A car bomb exploded outside the U.S. base at Bagram on Wednesday, injuring three contractors and underscoring the fragile security situation that has led Afghanistan’s elections commission to reject a demand by President Hamid Karzai to move up presidential elections to April or May.

The commission issued its ruling Wednesday at a Kabul press conference, in which they affirmed that security and weather conditions would not allow a vote to be held before August.

Afghanistan’s constitution requires that Karzai step down on May 21 when his five year term ends. The constitution also calls for elections to be held 30 to 60 days before a presidential term ends.

Karzai’s political opponents have said they will not recognize his authority after May 21, and may attempt to appoint an interim president selected from the parliament. Karzai’s demand for earlier elections was viewed as a way to respond to his critics, though it would be an enormous challenge to move the elections up to this spring.

NATO and western officials have supported the August date.

Wednesday’s bomb attack at Bagram Air Base occurred "outside the perimeter of the base when a vehicle exploded near an entry control point," a U.S. military statement read.

"The driver, who was carrying explosives, abandoned the vehicle before it detonated," the statement says. "The explosive he was carrying detonated as he ran away from the vehicle. The attacker was killed in the second explosion. It is unknown if there were additional personnel in the vehicle."

No military or Afghan casualties were reported. The contractors were treated at the base hospital for what officials called "minor injuries."

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