The US and nine EU and Nato countries have discussed strengthening security in central and eastern Europe amid renewed focus on Russia's actions in Ukraine and elsewhere in the region.
US president Joe Biden spoke via video-link with leaders from the nine states on the eastern flank of the EU and Nato as part of an online summit hosted by Romanian president Klaus Iohannis and visiting Polish counterpart Andrzej Duda.
Mr Biden "underscored his commitment to rebuilding alliances and strengthening trans-Atlantic relationships, he conveyed his desire for closer co-operation with our nine allies in central Europe and the Black Sea regions...and he also expressed his support for enhancing Nato's deterrence and defence posture", White House spokeswoman Jen Psaki said of Monday's talks.
The summit came as Nato began war games involving more than 28,000 troops from 26 nations conducting drills in a dozen countries, just weeks after Russia reportedly moved tens of thousands of troops towards eastern Ukraine and into occupied Crimea, sparking fears of all-out war between the neighbours.
“Nato must continue to strengthen its defence and deterrence posture especially on the eastern flank, from . . . the Baltic Sea to the Black Sea,” said Mr Iohannis.
"This is why I have argued, including in discussions with President Biden, for an increase of allied military presence in Romania and...the south of the eastern flank."
Nato members Romania, Bulgaria and Turkey are concerned by a major build-up of Russian weaponry in Crimea since the Kremlin annexed the Black Sea peninsula in 2014, after a pro-western revolution ousted Moscow-backed leaders in Ukraine.
Russia ended its snap combat readiness drills late last month and sent thousands of troops back to base, but Kiev, Washington and Nato warn that much of their heavy weaponry remains within swift striking distance of Ukraine.
“There is no doubt that the situation there is very difficult, that Ukrainian territory is occupied,” Mr Duda said alongside Mr Iohannis.
"Neither Europe nor the world can take their eyes off this part of our continent...We must all absolutely support Ukraine on the one hand, but on the other hand we must also guard our security, because this is our eastern flank, both for Romania and for Poland. "
Moscow's strained relations with most of the so-called Bucharest Nine deteriorated further amid recent allegations that Russian intelligence officers were behind explosions at arms depots in the Czech Republic and Bulgaria, two members of the group.
In response, Prague told dozens of Russian diplomats and other embassy staff to go home, and Slovakia, Estonia, Latvia and Lithuania – also members of the Bucharest Nine – expelled Russian diplomats in solidarity.
Leonid Slutsky, chairman of the foreign affairs committee of Russia's lower house of parliament, described reports on Monday that Prague may sue Moscow for compensation for the explosions as "absurd".