KABUL, Afghanistan - The Afghan government has postponed parliamentary elections for four months, citing security concerns, logistical problems and a lack of enough money from the country's international partners to pay for them.

The delay pushes the vote to September, instead of May as originally planned. The U.S. and its allies urged the Afghans to use the extra time to reform the electoral system to avoid a repeat of last year's fraud-marred vote, which undercut international support for President Hamid Karzai and threatened the Obama administration's strategy for reversing the tide of the war.

Another flawed balloting would make it difficult for Western leaders to rally support for the war among their own constituencies as U.S. and NATO casualties rise. Three American service members were killed Sunday in two separate bombings in southern Afghanistan.

That brings to 25 the number of American deaths in Afghanistan so far this month, compared with 14 for the whole of January last year.

The Independent Election Commission, whose chairman is appointed by the president, said it needed about $50 million from the international community to pay for the parliamentary election, budgeted to cost $120 million.

That money has not come through in time to hold the vote as planned on May 22, according to commissioner Fazel Ahmed Manawi. He also attributed the delay to security concerns, logistical challenges and the need to improve the election process at a news conference to announce the decision.

The vote will be held on Sept. 18, Manawi said.

U.S. and other international officials had made clear they would not pay for another election without a top-to-bottom overhaul of the electoral system in the wake of last August's balloting. U.N.-backed auditors threw out more than one million votes cast for Karzai, prompting a runoff set for November.

Karzai was proclaimed the winner only after his last remaining challenger, ex-Foreign Minister Abdullah Abdullah, dropped out, saying he was not confident the second-round balloting would be any cleaner than the first.

Widespread electoral fraud tarnished the image of Karzai in Washington and other Western capitals, casting doubt on President Barack Obama's strategy of working with a strong and credible Afghan government to win back support from the Taliban and its allies.

Karzai left Afghanistan on Sunday for Turkey en route to London for a major international conference Thursday aimed at shoring up support for the Afghan government.

An Afghan official said the government has $70 million left over from the previous election, but donor countries wouldn't provide the rest until they complete a thorough assessment of how the money was spent last year.

Following the election commission announcement, the U.S. Embassy said it respects and would work with the Afghans "to pursue election reform for the upcoming parliamentary elections and for the long term."

Canada's ambassador to Afghanistan, William Crosbie, said it was important to address the deficiencies in the presidential election before holding another vote.

"We encourage the Afghan government and the Independent Election Commission to set the necessary conditions for parliamentary elections that are credible, secure and inclusive," he said in a statement.

The U.N. said the decision would allow time to prepare for the vote and to improve the electoral process based on lessons learned from past votes. "This would have been extremely difficult to do by the original date," it said in a statement.

Canada's ambassador to Afghanistan, William Crosbie, said it was important to address the deficiencies in the presidential election before holding another vote.

"We encourage the Afghan government and the Independent Election Commission to set the necessary conditions for parliamentary elections that are credible, secure and inclusive," he said in a statement.

Some countries also were concerned that having to guard polling stations in May would be a distraction for the 37,000 U.S. and NATO reinforcements being sent to Afghanistan to try to turn back the Taliban.

Sunday's bombing brought to five the number of American service members killed in bombings over two days in southern Afghanistan, the Taliban heartland. The first explosion killed two troopers and one died in a second blast, NATO said in separate statements.

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Associated Press writers Heidi Vogt, Kim Gamel and Rahim Faiez in Kabul and Noor Khan in Kandahar contributed to this report.