KANO, Nigeria - Nigeria's bitterly contested presidential election went down to the wire Monday as the national election commission began announcing results. With almost a quarter of the ballots counted, the election was on a knife edge, with about 2.3 million votes apiece for President Goodluck Jonathan and his rival, former military ruler Muhammadu Buhari.

In the final hours of the count, U.S. Secretary of State John Kerry and British Foreign Secretary Philip Hammond took the unusual step of warning of "disturbing indications" of possible rigging of the results.

Voting in Nigeria's election saw delays and glitches with new electronic technology, although no one was prevented from voting. There also were scattered incidents of violence, ballot-box theft, and intimidation across the country.

But the real fears were always for the postelection period, after past elections in which disputed results led to deadly rioting by the unhappy supporters of losing candidates.

The warning by Kerry and Hammond of possible electoral fraud echoes accusations of rigging by local politicians from both parties, and underscores the risk of a likely disputed result, potentially triggering violence.

"So far, we have seen no evidence of systemic manipulation of the process. But there are disturbing indications that the collation process -where the votes are finally counted - may be subject to deliberate political interference," a joint statement said.

"The governments of the United States and the United Kingdom would be very concerned by any attempts to undermine the independence of the Electoral Commission (INEC), or its chairman, Professor (Attahiru) Jega, or in any way distort the expressed will of the Nigerian people," the statement warned.

In January, Kerry met Jonathan and Buhari and issued a tough warning that any Nigerian leader who promoted violence during the election would be denied a U.S. visa in the future.

Before the vote, Jonathan and Buhari signed the so-called Abuja accord, an agreement to refrain from any comments before, during, or after the election that could incite violence.

However, they signed a similar deal before the 2011 election and it did not prevent violence in the north after Buhari rejected the election results. Hundreds of his supporters rioted, and in some areas mobs dragged people from their homes and killed them. At least 800 people died, despite the fact that international observers saw the vote as one of the country's fairest.