With the critical state of Ohio too close to call, election officials are focussing on as many as 250,000 provisional ballots, which could determine the outcome of the U.S. presidential election.

Provisional, or backup, ballots are cast by those who find they are not listed on the voter rolls, or whose eligibility is somehow in question.

The ballots were allowed under a U.S. federal law passed after the 2000 election morass. After they are marked, they are set aside and evaluated after the election -- something that could take 11 days or longer to resolve.

When asked: "Who won Ohio?," Secretary of State Kenneth Blackwell told an early morning news conference it was simply too soon to tell.

"We'll know that when all the ballots are counted," said Blackwell, who is the state's chief election officer.

With so much at stake, and so many ballots in play, the Democrats were conceding nothing to the Republicans led by George W. Bush.

"The vote count in Ohio has not been completed," said Mary Beth Cahill, manager of the John Kerry campaign.

"There are more than 250,000 remaining votes to be counted. We believe when they are, John Kerry will win Ohio."

Not so, according to Bush's advisers, who told the president he would win Ohio.

Blackwell earlier told CNN there are at least 120,000 provisional ballots. But the Kerry campaign said the number could be as high as 250,000.

Meanwhile, Gov. Bob Taft, in an interview with CNN, said he could delay the final tabulation of the Ohio vote.

"It's possible we won't know the outcome for a period of 10 days," he said.

Blackwell did open the door to the possibility of calling the election results sooner than 10 days if Bush or Kerry's tally meet a specific requirement.

"If, in fact, the margin of victory for one candidate or the other is greater than the number of provisional ballots, then you can have a reasonable guess who won the election," Blackwell said.

But he cautioned the public and the media to take election projections in the coming days with a grain of salt.

"Everyone will be projecting and saying the unofficial count in Ohio has this candidate winning," he said.

By the early morning hours, 93 per cent of the precincts in Ohio had been counted. There was about a 100,000-vote difference, with Bush at 51 per cent and Kerry at 49 per cent.

As well, serious voting delays were being reported.

CTV's Lisa LaFlamme, reporting from a polling station in Columbus, Ohio, said some people were still waiting at 2 a.m. ET for their chance to vote, more than six hours after the polls had been due to close. Some people had been waiting as long as 16 hours.

As the election got underway, voter watchdog groups were sounding possibly alarms about delays in knowing the final outcome.

New rules, new voters and a tight presidential contest combined to create "a recipe for problems" and the likelihood that results won't be known for weeks, said Sean Greene, who was assigned to watch Cleveland polls for the Election Reform Information Project, a nonpartisan research group on election reform.