NATO Secretary-General Anders Fogh Rasmussen on Friday accused Russia of an “incursion” into Ukrainian territory, according to reports.
"Last night we saw a Russian incursion, a crossing of the Ukrainian border," he told reporters in Copenhagen, Denmark, according to Reuters.
"It just confirms the fact that we see a continuous flow of weapons and fighters from Russia into eastern Ukraine and it is a clear demonstration of continued Russian involvement in the destabilization of eastern Ukraine," Rasmussen said.
Rasmussen, though, was careful not explicitly to call Thursday night’s action an invasion.
The development comes as the Ukrainian military was set on Friday to inspect Russian trucks carrying humanitarian supplies destined for a city in eastern Ukraine.
The U.S. and allies have expressed concerns that Russia could use the humanitarian convoy as a cover for sending military aid to pro-Russian separatist groups in eastern Ukraine or for an invasion.
Fears that Russia is planning to invade Ukraine have grown with Moscow massing troops on the border as the government in Kiev steps up its offensive against the rebels.
Andriy Lysenko, spokesman for Ukraine’s security council, also claimed that Russian military vehicles had crossed into Ukraine, The Associated Press reports.
Russia, however, has denied that military vehicles entered Ukraine. Officials told The New York Times the trucks were for a private business and not the military.
British Foreign Secretary Philip Hammond said Friday he is “very alarmed” by the incursion, according to BBC News.
“If there are any Russian military personnel or vehicles in eastern Ukraine they need to be withdrawn immediately or the consequences could be very serious," he said.
The British foreign ministry has summoned Russia’s ambassador to the United Kingdom to “clarify” the situation.
Last week, Defense Secretary Chuck HagelChuck HagelLobbying World The US just attacked Syria. So where's Congress? Senators tear into Marines on nude photo scandal MORE said a Russian invasion into Ukraine is “a reality, it’s a threat, it’s a possibility,” citing Russia’s troop build-up along its border.
On Monday, State Department deputy spokeswoman Marie Harf suggested an invasion could happen at any moment.
“They’ve deployed a sizeable military force to the Ukrainian border. This force is capable of invading Ukraine on very short notice if Russia were to elect to do so. A large portion of this force is deployed only a few kilometers from the border,” she said.
The U.S. and European allies tightened sanctions on a number of Russian energy and financial firms after the crash of a Malaysian passenger jet, believed to be taken down by a surface-to-air missile fired by rebels.
The Obama administration has warned that an invasion would invite additional sanctions.