Ukraine should be able to choose its own alliances, regardless of what other countries think, US Vice-President Joseph Biden has declared in Kiev, according to BBC.
He appeared to be referring to Russia, which has made clear its opposition to Ukraine`s hopes of joining Nato.
Mr Biden, visiting Ukraine and then Georgia, is reiterating US support for those countries, amid fears it could be shifting its priorities towards Russia.
But Mr Biden said a thaw with Russia "will not come at Ukraine`s expense".
`No sphere of influence`
Following talks with Ukrainian President Viktor Yushchenko, he said the US would continue to support his policy of moving closer to the EU and the US, and away from Russia.
"If you choose to be part of the Euro-Atlantic integration, which I believe you have, then we strongly support that," Mr Biden said.
"We do not recognise - and I want to reiterate it - any spheres of influence," he said.
"We do not recognise anyone else`s right to dictate to you or any other country what alliance to belong to or what relationship to have."
Nato has promised eventual membership to Ukraine and Georgia, but has not given either a potential entry date.
Mr Obama has repeated his intention to "reset" relations with Russia, and visited Moscow last week, signing deals on nuclear disarmament.
But he also repeated that the US supported Georgia over its war with Russia last August, and would not accept the independence of the breakaway regions of South Ossetia and Abkhazia, as Russia had.
During Tuesday`s talks Mr Yushchenko called for US investment in Ukraine`s gas transport infrastructure.
Ukraine is a key transit country for Russian gas exports to Europe, but fell out with Russia this year over prices, leading to shortages in Europe.
The president is hoping to get outside investment to help upgrade the Ukrainian network and improve Europe`s energy security.
He told Mr Biden the US was seeking "as many investors as possible, including US investors".