By Andrius Sytas and Joanna Plucinska VILNIUS/WARSAW (Reuters) -U.S. President Joe Biden on Thursday promised Central European NATO members more military support as concern grows over a Russian troop build-up on the border with Ukraine, Lithuania's presidential advisor said. Biden also reassured the allies that Washington would not reach any agreement with Russia about the region behind their backs, advisor Asta Skaisgiryte told reporters. The U.S. president had spoken by phone to the leaders of NATO countries along the alliance's border with Russia, Belarus and Ukraine - Estonia, Latvia, Lithuania, Poland, the Czech Republic, Slovakia, Hungary, Romania, Bulgaria. He said additional reassurance elements are possible in these countries, and additional military capabilities, Skaisgiryte said, without naming possible locations. Russia has amassed troops on its border with Ukraine, where Kremlin-backed rebels have been fighting the Kyiv government, raising fears that it might be preparing to invade. Russian President Vladimir Putin has denied he intends to attack Ukraine, but he has bridled against what he sees as NATO's eastward expansion and the deployment of military hardware close to its border. On the leaders' call, Polish President Andrzej Duda pressed Biden to ensure that talks on how to handle any potential Russian aggression were not carried out solely in a small circle of countries, a Polish official said. Before a call with Putin on Tuesday, Biden consulted with the leaders of Britain, France, Germany and Italy. Four countries can't speak on behalf of NATO, the Polish official told Reuters. Duda also told Biden that the United States should not shy away from increasing its troop presence on NATO's eastern flank, the Polish official said. In Washington, a senior Biden administration official said Biden reiterated the U.S. commitment to the NATO charter which says an armed attack against one member shall be considered an attack on them all. Biden also spoke to Ukraine President Volodymyr Zelenskiy and made clear that the United States and its allies would make no decisions or discussions about Ukraine without Ukraine,' the White House said. NO CONCESSIONS Biden's phone call with the eastern leaders followed his promise of high-level meetings with Russia and major NATO allies to discuss Moscow's concerns and to try to cool tensions. The senior U.S. official said the United States was prepared to discuss security issues with Moscow but that Biden made no concessions when he spoke to Putin on Tuesday. Biden said that Ukraine was a sovereign country which must decide its own future, Lithuania's Skaisgiryte said. He also warned Putin of very serious consequences, coordinated with the European partners if Russia invades Ukraine, she said. Russia kept up a barrage of hostile rhetoric towards Ukraine on Thursday and compared the crisis there to the most dangerous moment of the Cold War. Lithuanian president Gitanas Nauseda told Biden that a permanent U.S. troop presence in his country would be most effective for deterrence and security in the region. (Reporting by Andrius Sytas in Vilnius, Joanna Plucinska in Warsaw and Steve Holland and Trevor Hunnicutt in Washington; Editing by Angus MacSwan)