Ukraine's hopes of becoming a member of Nato has the support of the US but it has not yet met the requirements needed to enter, said President George W Bush.
Mr Yushchenko (left) wants to build stronger ties with the US
Mr Bush was speaking after talks with Ukrainian President Viktor Yushchenko, on his first official visit to the US.
He backed Ukraine's hope of joining the World Trade Organisation by the end of 2005 and the end of trade sanctions.
Ukraine has drawn closer to the US since Mr Yushchenko, a pro-western liberal, took office in January.
Mr Bush described Ukraine's bid for membership of Nato as "a path and we want to help Ukraine get on that path as quickly as possible".
But he said membership was "not a given".
"In other words, there are things that the Ukrainian government must do in order to satisfy the requirements to be considered for Nato," he told a news conference in Washington.
Mr Bush said the US was a "strong friend" of Ukraine.
He has repeatedly cited Ukraine's bloodless Orange Revolution - that brought Mr Yushchenko to power - as an example of his global vision of spreading democracy and freedom.
But, in his press conference, he glossed over suggestions that he was disappointed by Mr Yushchenko's decision to withdraw Ukrainian troops from Iraq.
"The president made clear to me in my first conversation with him that he campaigned on the idea of bringing some troops out.
"He is fulfilling a campaign pledge. I fully understand that," Mr Bush said.
Nevertheless, the White House is trying to hide its disappointment at Mr Yushchenko's decision to pull out troops, the BBC's Jonathan Beale reports from Washington.
Mr Yushchenko spoke of a new era of strategic partnership with the US, and said he hoped trade sanctions on Ukrainian goods could soon be lifted.
The US has promised economic aid to help Ukraine root out corruption and implement reforms.
Mr Yushchenko was expected to reassure the US that his administration will tighten controls against weapons smuggling.
On Wednesday, the Ukrainian president will address a joint session of Congress, an honour reserved for Washington's closest allies.