Mr Obama said that in a phone call, he had also asked Mr Karzai to intensify efforts to eradicate corruption.

He was speaking after the Afghan president had been declared winner of August's fraud-marred election.

Earlier, poll officials scrapped a planned run-off following the withdrawal of Mr Karzai's challenger.

Mr Obama said he had congratulated Mr Karzai on his re-election.

UN Secretary General Ban Ki-moon, who is in Kabul, also congratulated Mr Karzai.

He said Afghanistan's troubled election had been among "the most difficult the United Nations has ever supported".