The Afghan election commission rejected President Hamid Karzai's request to move the presidential elections to the spring, saying the country won't be safe enough or have enough money by then to hold a vote.

The commission's decision came Wednesday as a car bomb exploded outside the main U.S. base at Bagram, underscoring the shaky security situation the country faces as a resurgent Taliban militia increases its attacks.

Karzai had asked the commission to move the elections from Aug. 20 to spring, but the commission said it could not because of bad spring weather, lack of funds, security issues and logistical problems like the distribution of ballots.

Election officials say high mountain snows would prevent many thousands of Afghans from voting in the spring.

"Weather, funding, operational challenges and logistical issues and of course security remain the same and there have not been solutions to these problems," Azizullah Lodin, the head of the Afghan election commission, told a news conference.

Adhering to the earlier deadline would be "impossible," Lodin said.

Lodin said that $223 million is needed to hold the vote, and that his commission has received nothing except a pledge of $100 million from the international community.

The U.S. has called for an August vote. Thousands more U.S. troops are expected to arrive by then and could help with security. The U.N. welcomed the decision, saying the August vote would provide the time needed to make logistical and security preparations.

Karzai last week asked the commission to re-examine the date to see if it could be held in line with the Afghan constitution, which says Karzai must step down on May 22 and that elections must be held 30 to 60 days before that.

A late summer vote means the country will face a three-month gap between the end of Karzai's term and the election.

Lawmakers have said they won't recognize Karzai as president beginning May 22, which could throw the country into constitutional crisis. Some lawmakers have called for a caretaker president to replace Karzai.

Ashraf Ghani Ahmadzai, a former finance minister who said he will challenge Karzai, welcomed the decision.

"The question that arises is how power should be exercised between May 22, when the term in office of the president ends and the date when the newly elected president swears in," Ghani told reporters during a gathering of all major opposition figures in Kabul.

The constitution gives Karzai at least two options to avoid a crisis: Call a state of emergency that would extend his presidency but which would need lawmakers' approval, or call a loya jirga _ a grand meeting of Afghan leaders _ to negotiate an agreement.

Either option would likely require a political deal with his opponents.

The car bomb blast outside Bagram wounded three civilian contractors working for a U.S. company, but it wasn't immediately clear what nationalities the three were, said Lt. Col. Rumi Nielson-Green, a U.S. spokeswoman.

The attack occurred near a parking area where truck drivers bringing in supplies gather. The driver of the car bomb abandoned the vehicle before it detonated, but the attacker was also carrying explosives which detonated, killing him, the U.S. military said.

A Taliban spokesman, Zabiullah Mujahid, claimed responsibility for the blast soon after the attack.

In the south, a Canadian general said a roadside bomb blast in Kandahar province late Tuesday killed three Canadian soldiers.

Militant attacks are killing U.S. and NATO soldiers at a record pace this year. In the first two months of the year, 29 U.S. soldiers died in Afghanistan, compared with eight in the first two months of 2008.

The U.S. is sending 17,000 additional U.S. forces to Afghanistan this year to bolster the record 38,000 in the country.

Associated Press reporters Amir Shah and Fisnik Abrashi in Kabul contributed to this report.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.