His comments come after a surge of cease-fire violations in eastern Ukraine, where Russia-backed separatists and Ukrainian forces have been locked in a conflict since Moscow’s 2014 annexation of Crimea.

 Russia’s defence minister Sergueï Choïgou said the moving of troops and armoury to the border area was for military exercises.

“Russia has moved thousands of combat troops to Ukraine’s borders, the largest massing of Russian troops since the illegal annexation of Crimea in 2014,” said Stoltenberg. “Over the last days, several Ukrainian soldiers have been killed in eastern Ukraine.

“Russia must end this military buildup in and around Ukraine, stop its provocations and de-escalate immediately.”

Stoltenberg will be joined by US Secretary of State Antony Blinken, who is expected in Brussels for talks on Ukraine and other issues.

Blinken will be joined in the Belgian capital by US defence secretary Lloyd Austin for additional consultations on Wednesday, said the State Department.

G7 calls on Russia to ‘cease provocations’

Late on Monday, The G7 group of countries also called on Russia to “cease provocations”.

The foreign ministers of the G7 – Canada, France, Germany, Italy, Japan, the UK, and the US, plus the High Representative of the European Union – said they were “deeply concerned” about the military build-up, and urged Russia to “cease its provocations and to immediately de-escalate tensions”.

Euronews journalists visited Mayorsk, recently, where Ukrainian soldiers reported an increase in fighting and the deaths of four people from their army nearby in shelling a week ago.

Ukraine’s president, Volodymyr Zelenskyy, says 26 Ukrainian soldiers have died this year.

Ukraine has accused Russia of massing more than 80,000 troops near its eastern border and in Crimea, which Moscow annexed in 2014.

According to Kyiv, the pro-Russian separatists also have 28,000 fighters and more than 2,000 Russian military advisers and instructors in the territory they have controlled since 2014 in the east of the country.

“These large-scale troop movements, without prior notification, represent threatening and destabilising activities,” said the G7 foreign ministers in a statement.

“We reaffirm our unwavering support for the independence, sovereignty and territorial integrity of Ukraine within its internationally recognised borders,” the statement added.

The US warned Moscow on Sunday against any aggression in Ukraine.

“There will be consequences,” said Blinken.

Listen to Euronews’ interview with Bruno Lété of the German Marshall Fund by clicking on the media player above.