An explosion set a U.N. vehicle on fire in eastern Afghanistan (search), injuring its Afghan driver, and three suspected militants were killed when a mine exploded prematurely, officials said Sunday.

The vehicle fire on Saturday morning came just days after the killing of three U.N. (searchelection workers, including two British security consultants, in another eastern province.

But the world body said it was pressing on with preparations for the September vote, seen as key to bringing democracy to Afghanistan after a quarter-century of war.

The U.N. vehicle was hit near Grabawa, a village in Khogyani district of Nangarhar province about 60 miles south of the capital, Kabul (search), U.N. spokesman Manoel de Almeida e Silva said.

Four Afghan election staff traveling in the vehicle were unhurt, and the driver was released from a hospital later Saturday after treatment for minor injuries, Almeida e Silva said.

"They all managed to get out of the car before it was engulfed in flames," he said.

Almeida e Silva said investigators were still trying to establish if the explosion, which burst open the vehicle's fuel tank, was caused by a mine or a remote-controlled bomb.

"We don't know whether or not this vehicle yesterday was targeted," the spokesman said.

But Gen. Mohammed Yunus Noorzai, the Nangahar police chief, blamed anti-government militants for what he said was a deliberate attack.

"We suspect Taliban or Al Qaeda were behind this incident," Noorzai told The Associated Press, but offered no information to back up his assertion.

In Kabul, the Defense Ministry said three suspected militants were killed early Thursday morning as they tried to lay a remote-controlled mine on a road near Gardez, the capital of Paktia province some 60 miles south of Kabul.

"It seems that one of them was busy setting it up when it went off," ministry spokesman Gen. Mohammed Zahir Azimi said. A fourth suspect was injured, he said, but gave no details.

The two British contractors were killed along with their Afghan translator on Wednesday in Nuristan province.

The Afghan government has yet to release the results of an investigation into the killings, which the two men's London-based firm blamed on "local bandits." A spokesman purporting to speak for the Taliban claimed responsibility for the attack.

Almeida e Silva said that incident had led to a delay in registration plans for Nuristan, but no change in the plans elsewhere. Some 2 million out of an estimated 10 million Afghans have now been registered, he said.

"The plans for voter registration and the activities continue as scheduled," he said.

More than 300 people have died in violence across Afghanistan so far this year, casting doubt on whether Afghanistan is stable enough for elections — where U.S.-backed President Hamid Karzai is expected to triumph.