Republican challenger Mitt Romney conceded defeat in the US election early Wednesday, telling supporters that he had called President Barack Obama to congratulate him on his victory.

"His supporters and his campaign also deserve congratulations," he said, in a brief address at his election watch party in Boston.

"I wish all of them well but particularly the president, the first lady and their daughters."

It was a quick, underwhelming end to an 18-month campaign that began on a farm in New Hampshire, survived brutal Republican infighting during the party primaries early this year and a barrage of negative attack ads by the Obama camp, and rose to give the incumbent a serious scare weeks before the election.

Romney was neck and neck with the president for a considerable part of the campaign, but despite repeated trips to swing states like Colorado Ohio, Florida, Virginia and Iowa, Obama held on to leads in the battlegrounds, which eventually became the challenger's undoing.

As of 1:20 am Wednesday, Obama won 303 electoral votes, compared to 203 for Romney, with only Alaska and the key battleground of Florida outstanding.

"This is a time of great challenges for America and I pray that the president will be successful in guiding our nation," he said, to modest cheers and applause.

Several members of Romney's senior staff stood next to the stage, many stone-faced and somber, as the Republican nominee addressed his supporters.

Romney returned to a theme that he began injecting into his stump speeches in the closing two weeks of the campaign: the need for greater bipartisanship in Washington.

"At a time like this we can't risk partisan bickering and posturing," he said.

"Our leaders have to reach across the aisle to do the people's work and we citizens have to rise to the occasion."

Romney also thanked his wife Ann, his tireless surrogate on the campaign trail whom he called "the love of my life."

"She would have been a wonderful first lady," he mused, to loud applause.

Romney's comments were brief and basic, and it was not immediately clear if he had written a concession speech.

Earlier in the day, when asked by reporters on his campaign plane whether he had two speeches ready to go for Tuesday night, he said he was confident of defeating Obama and had penned an 1,118-word victory speech.