According to EU top diplomat Josep Borrell, Russia has concentrated over 100,000 troops on Ukraine's border and in annexed Crimea.

Following a briefing by Ukraine's foreign minister, Mr. Borrell announced rising tensions in the region.

Mr. Borrell said on Tuesday (AEDT) that “the possibility of further escalation is evident,” but he declined to provide a source for the number, which he initially said was more than 150,000 before his services revised it down.

“This is the Russian army's largest military deployment on the Ukrainian border in history. When you send a large number of soldiers, it's obvious that it's a cause for concern,” he said.

Despite claiming that the military build-up on Ukraine's borders was the biggest ever, Mr. Borrell said no new economic sanctions or expulsions of Russian diplomats were expected for the time being.

The Pentagon in Washington said that Russia's military build-up was larger than in 2014 and that it was unclear if it was for training purposes.

The Russian build-up was in the tens of thousands, according to a US official who spoke on the condition of anonymity, but he was unaware of intelligence that pointed to more than 150,000 Russian troops.

After speaking to EU foreign ministers, Ukrainian Foreign Minister Dmytro Kuleba called on the EU to place new sanctions on Russia.

Tensions between Moscow and Kyiv have been that as a result of the military build-up and clashes between the army and pro-Russian separatists in eastern Ukraine.