Ukrainian President Viktor Yushchenko has criticised new Prime Minister Viktor Yanukovych for suggesting Kiev was not ready to join Nato.
An anti-Nato slogan featured in Mr Yanukovych's poll campaign
The president said Mr Yanukovych's belief that public opinion opposed the move was "wrong, does not meet national interests and must be corrected".
His prime minister called for "a pause" on Thursday after talks with top Nato and EU officials in Brussels.
Mr Yushchenko has pushed for membership of Nato following his election in 2005.
Mr Yanukovych was named PM in August by his arch-rival Mr Yushchenko, on condition that he followed his pro-Western agenda.
The move capped a dramatic comeback for Mr Yanukovych, who was ousted in Mr Yushchenko's "Orange Revolution" in 2004.
Mr Yushchenko told reporters his government had to abide by his stance of integration with Western institutions as the "foundation, the credo for foreign policy".
The comments came a day after Mr Yanukovych said full Nato membership had only limited support among Ukrainians, on his first visit to Brussels as prime minister.
He said Kiev was taking a pause "because of the political situation in Ukraine".
"But the time will come when a decision will be made... For the time being we are looking at enlargement of our co-operation with Nato," he said.
Opposition to Nato membership is particularly strong in eastern and southern Ukraine - the electoral strongholds of Mr Yanukovych's party.
Russia has also voiced strong opposition to Ukraine joining Nato.
Kiev had earlier expressed hopes of joining the world's biggest defence alliance in 2008.
At the same time, Mr Yanukovych said Ukraine would continue reforms aimed at bringing the country closer to the EU.
EU External Relations Commissioner Benita Ferrero-Waldner said the 25-member bloc had no plans to offer Kiev membership "at this moment".
Instead, she suggested the two sides negotiate what was described as an enhanced agreement that would include a free trade pact.
Mr Yanukovych - who favours closer ties with Russia - was initially declared the victor in the 2004 presidential polls, but the result was then annulled by the Supreme Court, which ruled that the vote was fraudulent.
Mr Yushchenko was elected president in the re-run of the second round ordered by the court.
In March, Mr Yanukovych's Party of Regions polled the most votes in parliamentary elections, but failed to secure a majority.