WASHINGTON — President Trump has relaxed some of the rules for preventing civilian casualties when the American military carries out counterterrorism strikes in Somalia, laying the groundwork for an escalating campaign against Islamist militants in the Horn of Africa.

The decision, according to officials familiar with internal deliberations, gives commanders at the United States Africa Command greater latitude to carry out offensive airstrikes and raids by ground troops against militants with the Qaeda-linked Islamist group Shabab. That sets the stage for an intensified pace of combat there, while increasing the risk that American forces could kill civilians.

Mr. Trump signed a directive on Wednesday declaring parts of Somalia an “area of active hostilities,” where war-zone targeting rules will apply for at least 180 days, the officials said.

The New York Times reported the Pentagon’s request for the expanded targeting authority on March 12, and Gen. Thomas D. Waldhauser, the top officer at Africa Command, publicly acknowledged that he was seeking it at a news conference last Friday.

“It’s very important and very helpful for us to have little more flexibility, a little bit more timeliness, in terms of decision-making process,” General Waldhauser said. “It allows us to prosecute targets in a more rapid fashion.”

In a statement issued several hours after The New York Times first published news of the directive, Capt. Jeff Davis, a Pentagon spokesman, acknowledged that Mr. Trump had approved the Pentagon’s proposal to expand its targeting authority “to defeat Al Shabab in Somalia” in partnership with African Union and Somali forces.

“The additional support provided by this authority will help deny Al Shabab safe havens from which it could attack U.S. citizens or U.S. interests in the region,” he said.

Previously, to carry out an airstrike or ground raid in Somalia, the military was generally required to follow standards that President Barack Obama imposed in 2013 for counterterrorism strikes away from conventional war zones, like those in Afghanistan and Iraq.

Those rules, known as the Presidential Policy Guidance, required high-level, interagency vetting of proposed strikes. They also said that the target must pose a threat to Americans and that there must be near-certainty that no civilian bystanders would die.

Under the new guidelines, Africa Command may treat Somalia under less-restrictive battlefield rules: Without interagency vetting, commanders may strike people thought to be Shabab fighters based only on that status, without any reason to think that the individual target poses a particular and specific threat to Americans.

In addition, some civilian bystander deaths would be permitted if deemed necessary and proportionate. Mr. Trump’s decision to exempt much of Somalia from the 2013 rules follows a similar decision he made for parts of Yemen shortly after taking office.

The new directive for Somalia is another example of how the American military is accelerating the ways it carries out combat missions under the Trump administration, reducing constraints on the use of force imposed by the Obama administration.

As the fight against the Islamic State in Iraq and Syria has recently moved into the city of Mosul, civilian casualties have spiked. One American strike on March 17 may have killed scores of civilians, and human rights groups have questioned whether the rules of engagement were to blame.

While American commanders say the formal rules of engagement have not changed in Iraq, they acknowledge that the system for calling in airstrikes there has been accelerated. Gen. Joseph L. Votel, the commander of United States Central Command, said on Wednesday that the new procedures made it easier for commanders in the field to call in airstrikes without waiting for permission from more senior officers.

The loosening of the rules in Somalia comes against the backdrop of a broader, continuing Trump administration policy review about whether to scrap the 2013 rules altogether. The decision was described by officials familiar with the new directive who spoke on the condition of anonymity to discuss military planning

Luke Hartig, a former senior director for counterterrorism at the National Security Council during the Obama administration, said greater action could be helpful in dealing with a threat, pointing to the Obama administration’s decision last year to temporarily declare the region around Surt, Libya, an active-hostilities zone. That decision similarly permitted airstrikes that helped Libyan forces root out Islamic State militants.

But it also increases certain risks, he said.

“The downside is you risk potentially greater civilian casualties or potentially killing militants who are not part of our enemy,” Mr. Hartig said. He warned that such deaths could make local partners turn against the United States and fuel terrorist recruitment.

Mr. Trump’s decision to relax targeting limits in Somalia comes at a time of famine, which has increased the frequency of groups of people moving around, often while armed, in search of food and water — increasing the risk of mistaking civilians as Islamist fighters.

General Waldhauser said at the news conference that Africa Command had “war-gamed” the “significant” issues raised by that factor.

“It’s our responsibility to make sure that we don’t have any catastrophes and we don’t take out a group of people who is moving to find water or food,” he said. “So, we are very, very conscious of that.”

Defense Secretary Jim Mattis first presented the proposal to relax targeting limits in Somalia at a dinner with Mr. Trump about five days after his inauguration, according to officials familiar with internal deliberations.

At that same dinner, Mr. Mattis also presented proposals to similarly remove swaths of Yemen from the Obama-era targeting limits and carry out a raid against Yemen’s Qaeda branch. Mr. Trump, the officials said, immediately approved the two proposals for Yemen, while the National Security Council began a review of the Somalia proposal.

The review for Somalia was slowed, officials have said, by criticism of the raid in Yemen, which resulted in numerous civilian deaths, the death of a member of the Navy SEALs and the loss of a $75 million aircraft. Still, the Central Command, which oversees military operations in Yemen, has carried out a fierce campaign of airstrikes in Yemen.

The United States’ campaign against the Shabab in Somalia has been expanding over the last several years. That Islamist group is complex, with some factions focused on controlling Somalia, while others want to participate in external terrorist operations in line with Al Qaeda’s global war.

In 2013, the group carried out the attack at the Westgate mall, in Nairobi, Kenya, that killed more than 60 people and wounded more than 175. Since then, it has adopted more sophisticated forms of terrorism, including nearly bringing down a Somali airliner last February with a bomb hidden in a laptop computer.

To counter the Shabab, the United States has increasingly used Special Operations forces, airstrikes, private contractors and African allies. Hundreds of American troops now rotate through makeshift bases in Somalia, the largest military presence since the United States pulled out of the country after the “Black Hawk Down” battle in 1993. They have served as trainers and advisers to African Union and Somali government forces, and have sometimes participated directly in combat.

Against that backdrop, Mr. Trump’s escalation is less a break with his predecessor than an intensification of a trend that dates to Mr. Obama’s last year in power.

Last year, the Obama White House permitted the military to ramp up airstrikes in Somalia without always going through the high-level vetting process detailed in the 2013 rules. Instead, the military justified some strikes under an expansive interpretation of an exception for “self-defense” — including some that defended partner forces combating the Shabab even if no Americans were under direct threat.

And as The Times reported in November, the Obama administration — after years of internal debate — decided to designate the Shabab an “associated force” of Al Qaeda. That shored up the executive branch’s authority to wage war in Somalia by bringing the Shabab under Congress’s authorization to use military force against the perpetrators of the Sept. 11, 2001, terrorist attacks.

Even before the new relaxations of the rules, about 200 to 300 American Special Operations forces have been working with soldiers from Somalia and other African nations like Kenya and Uganda to carry out more than a half-dozen raids every month, according to senior American military officials. The Navy’s classified SEAL Team 6 has been heavily involved in many of these operations.

The Pentagon has acknowledged only a fraction of these missions. But even the publicly available information shows a marked increase in recent years. The Pentagon announced 13 ground raids and airstrikes in 2016, up from five in 2015, according to data compiled by New America, a Washington think tank. Those strikes killed about 25 civilians and 200 people suspected of being militants, the group found.