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Higgins' warm wishes to Obama

"My wife Sabina and I send our warmest personal wishes to you and your family as you begin your second term," President Higgins said in his statement.

"The very close and warm relationship between Ireland and the United States has, with your help and encouragement, prospered during your tenure. I am confident that, under your leadership, our bilateral relations will be further advanced during the next four years."

Shortly after he was declared the victor, Obama told ecstatic supporters the "best is yet to come" in a victory speech in his home city of Chicago.

Flanked by wife Michelle and daughters Malia and Sasha, Obama congratulated opponent Mitt Romney on a "hard-fought campaign".

The president told the crowd, who chanted "four more years" as he arrived on stage, that they had "lifted me up the whole way and I will always be grateful".

He heaped praise on his family, revealing he had never loved his wife Michelle more and commending his "smart, beautiful" daughters.

"Let me say this publicly, Michelle -- I have never loved you more. I have never been prouder to watch the rest of America fall in love with you, too, as our nation's first lady."

The president spoke warmly of his two daughters, telling them he was proud of them, but that "for now, one dog is probably enough".

Mitt Romney earlier conceded defeat, phoning the president to congratulate him on his victory.

He told his supporters they had given their all to the campaign and said he wished his Democrat opponent well.

Although polls put the two candidates neck and neck, Obama secured a convincing victory.

Under the electoral college system, which allots votes to states according to population, he has 303 while Romney is on 206, with Florida yet to declare: 270 votes are required for victory.

After it became clear the Republican candidate could not win, the president took to Twitter to tell followers "four more years".

In his victory speech, he praised "the spirit that has lifted this country from the depths of despair to the great heights of hope".

"Tonight, you, the American people, reminded us that while our road has been hard, we have picked ourselves up, we have fought our way back, and we know in our hearts that, for the United States of America, the best is yet to come."