KIEV, May 31 - A hard-won agreement between Ukraine's rival leaders to hold new elections this fall was cast into doubt Thursday as parliament ended its session hours ahead of a presidential deadline to pass legislation supporting the deal.

President Viktor Yushchenko and Prime Minister Viktor Yanukovych agreed Sunday to elect a new parliament Sept. 30, easing tension amid a persistent power struggle in the ex-Soviet republic, but efforts to pass laws governing the vote have foundered this week amid mutual recriminations.

Yushchenko, who had been calling for a much earlier vote before the compromise deal, said Thursday his allies would withdraw from parliament if the laws were not approved by midnight ??” a move he said would trigger a new election in two months.

"If a solution is not reached, my party and (Yulia) Tymoshenko's party will meet and formalize our withdrawal from parliament," Yushchenko said during a visit to Croatia, according to The Associated Press report.

"Then elections will take place automatically in 60 days," he added.

But parliament, dominated by Yanukovych's majority coalition, ended its session without approving the legislation. Coalition members vowed to return Friday.

The head of Yushchenko's group in parliament, Vyacheslav Kyrylenko, accused the coalition of violating the agreement and said it "must take all responsibility for future development of events on itself".

He also said that 172 opposition lawmakers had registered their resignations at the parliamentary secretariat, an initial step toward quitting parliament. The resignation of 151 lawmakers is required to dissolve parliament and force elections in 60 days, but a leading Yanukovych ally suggested his camp would resist any attempts to hold a vote in that time frame.

"It is impossible to hold any early elections if the package of bills is not adopted by parliament," said lawmaker Taras Chornovil of the Regions Party.

Ukraine has been embroiled in a political crisis since Yushchenko issued on April 2 a decree to dissolve the parliament and to call early elections. Yanukovych and his governing coalition called the order illegal and appealed against it to the Constitutional Court.

Sunday's pre-dawn agreement eased concerns the standoff could escalate into violence after Yushchenko fired the prosecutor-general and the Internal Affairs Minister Vasyl Tsushko sent police to prevent him from being evicted from his office.

Yushchenko then claimed control of ministry's forces and sent some to the capital, although Tsushko refused to recognize the order.

Yushchenko and Yanukovych were bitter rivals in Ukraine's 2004 presidential election. Yanukovych was declared the winner of a fraud-riddled vote that sparked mass protests known as the Orange Revolution.

Yushchenko won a court-ordered rerun of the balloting, but Yanukovych returned to prominence last year when his party won the largest share of seats in parliament. (ap/ez)