Ukrainian President Viktor Yushchenko is to address the nation on how he intends to end a political crisis over the formation of a new government.
Mr Yushchenko wants his rivals to commit to a pro-Western agenda
A midnight (2100 GMT) deadline passed without word from the pro-Western leader who must decide whether or not to accept his arch-rival as premier.
Otherwise he must dissolve the parliament and call new elections.
It is seen as one of his toughest challenges since being brought to power by mass street protests in 2004.
Ukraine has been in political turmoil since a parliamentary ballot in March in which no party won a majority.
However, the strongest contender for the prime minister's post appears to be Viktor Yanukovych, the disgraced loser of the presidential election in which claims of mass vote-rigging sparked the popular protests.
"President Viktor Yushchenko will announce his final decision concerning his subsequent actions in a televised address today," his spokeswoman Iryna Gerashchenko said.
Her comments came as Mr Yushchenko was holding talks with party leaders and the parliamentary speaker to discuss the possibility of disbanding the parliament.
At the talks, Mr Yushchenko said that efforts to find a compromise had so far failed, without saying what he intended to do.
Ukrainian lawmakers have gathered for an emergency session of parliament, and some 2,000 demonstrators have been holding rival rallies near the building.
A coalition of three pro-western parties which backed the streets protests in 2004 - the so-called Orange Revolution - collapsed before it could form a government.
Mr Yanukovych made a strong comeback in the March elections
It was replaced by a coalition led by Mr Yanukovych which favours closer ties with Russia.
Approving Mr Yanukovych would be a humiliation for the president, the BBC's Helen Fawkes in Kiev says.
But it is not clear whether Mr Yushchenko has the right to reject the nomination, our correspondent says.
Over the last few days there have been intense talks about the possibility of the president's Our Ukraine party joining the coalition.
Mr Yushchenko has also been holding discussions with Ukraine's political leaders to negotiate an agreement to ensure his pro-Western policies will continue.
21/11/04 Yanukovych declared winner of presidential election - protests begin
3/12/04 Election annulled
11/1/05 Yushchenko declared winner of re-vote
8/9/05 Yushchenko sacks Tymoshenko government
26/3/06 Yanukovych party wins most votes in general election
On Tuesday, he had marathon talks with Mr Yanukovych, trying to persuade him to sign the so-called unity agreement.
The president is seeking concessions from Mr Yanukovych's Party of Regions on several divisive issues, including Ukraine's bid to join the EU and Nato and the status of the Russian language.
Mr Yushchenko has threatened to call new elections if a compromise cannot be reached, but says this option is a last resort.
On Wednesday, Mr Yanukovych said he did not expect parliament to be dissolved.
His coalition has warned that a dissolution of parliament will be illegal and they will ignore it.