Igor Popov, an aide to President Viktor Yushchenko, said in the Ukrainskaya Pravda daily that the government might declare an emergency and reschedule the election on May 30.

"The epidemic has radically changed the course of the election campaign," Popov was quoted as saying. "A ban on public events has affected opposition candidates."

He added that presidential hopefuls with government jobs can freely visit regions that are quarantined because of the flu, while opposition candidates cannot and "feel discriminated upon."

A total of 16 candidates have been registered so far, election officials said.

Ukraine's flu death toll has climbed to 109 since last week's outbreak in the western city of Ternopil. The World Health Organization says most flu cases in the country can be assumed to be H1N1.

Yushchenko and Prime Minister Yulia Tymoshenko, his former ally turned bitter foe, will face off for the presidency at the election.

The vote might overturn the results of the 2004 Orange Revolution that swept a pro-Western government to power in Ukraine.

Leading in the polls is Viktor Yanukovich, who was beaten in 2005 by Yushchenko, but has pulled ahead on a platform that emphasizes closer ties with Russia.