US President Barack Obama urged NATO on Wednesday to help strengthen Ukraine's military and said the alliance must leave the door open to new members to counter what he called Russian aggression.

Obama was speaking in Estonia, one of three ex-Soviet Baltic states bordering Russia that fear separatist rebellion in eastern Ukraine could herald problems for them. All three have sizeable Russian minorities and rely on Russian fuel deliveries.

United front in Estonia ... President Obama - joined by the leaders of Baltic States, from left, Estonia President Toomas Hendrik Ilves, Latvia President Andris Berzins and Lithuanian President Dalia Grybauskaite - said 'the door to NATO membership will remain open'. Credit:AP

Obama, who will attend a NATO summit in Wales on Thursday, accused Russia of a 'brazen assault' on Ukraine, which for five months has been fighting pro-Russian separatists that Moscow describes as a defence force warding off Ukrainian aggressors.

"NATO must make concrete commitments to help Ukraine modernise and strengthen its security forces. We must do more to help other NATO partners, including Georgia and Moldova, strengthen their defences as well," he said a speech to a packed concert hall in the Estonian capital.