Moscow has around 100,000 troops stationed close to Ukraine's frontier and Western leaders were preparing for telephone consultations amid fears of an invasion.

A total of 1,000 additional US troops meant to bolster NATO's eastern flank are arriving in Romania this week, while more allied troops and equipment are also being sent to other eastern European countries.

"There is a real risk for a new armed conflict in Europe. The number of Russian troops are going up, and the warning time is going down," Stoltenberg told reporters after visiting the Mihail Kogalniceanu base near the Black Sea with Romanian President Klaus Iohannis.

"At the same time we don't have any certainty, and therefore we continue to reach out to Russia to call on them to deescalate and to engage in good faith in political dialogue with NATO and NATO allies," he added.

US President Joe Biden announced last week that he was sending 1,000 soldiers to Romania and 2,000 to Poland.

Those arriving in Romania are being transferred from a base in Germany.

France's President Emmanuel Macron has also said he's ready to send hundreds of troops to Romania under NATO command.

Romania, a NATO member since 2004, already hosts around 900 US troops, as well as 140 Italian and 250 Polish troops.

Bulgaria, Romania's southern neighbour, said four Spanish Eurofighter Typhoon fighters arrived Friday as part of efforts to secure NATO's eastern flank.

The aircraft with 130 Spanish personnel will be stationed at the southern Graf Ignatievo air base and "perform enhanced air policing tasks" with the Bulgarian air force until March 31, the Bulgaria's defence ministry said in a statement.

As a NATO member, Bulgaria is required to keep at least one squadron of 12 aircraft in good fighting order.

But the inability to maintain enough of its ageing fleet of Soviet-built MiG-29 fighters prompted Sofia in 2016 to authorise NATO to help protect its airspace.

Bulgaria's Defence Minister Stefan Yanev confirmed on Thursday an expected delay in the delivery of eight new F-16 fighters that the country ordered from US manufacturer Lockheed Martin in mid-2019.

Amid the Ukraine crisis, Bulgaria has offered to establish on its territory a battle group of 1,000 troops, with support from other NATO allies.