A car exploded in a southern Afghan town on Wednesday, killing its three occupants and a passerby, in what appeared to be a bungled suicide attack, a provincial official said.

Security has been stepped up across Afghanistan in the run-up to Sept 18 elections that have been denounced by Taliban, who have been battling U.S. and government forces since their ouster in 2001.

The car was passing a policeman's house in the town of Girishk when it exploded but that was not believed to have been the target, said Mohammed Wali Alizai, spokesman for the Helmand provincial governor.

"It seems their aim was to carry out a suicide on attack on the Americans but it went off early," Alizai said.

"The car was completely destroyed, it's in pieces," he said.

The United States has 20,000 troops in Afghanistan, most of whom are focusing on election security, as are 10,000 NATO-led peacekeepers and tens of thousands of government forces.

The parliamentary and provincial elections are the final part of a plan meant to restore democratic government and stability in Afghanistan after 25 years of conflict.

PHOTO CAPTION

Emergency firefighting crew follows a U.S. cargo plane C-130 as it lands in Bagram Base, 50km north of Kabul, Afghanistan, Monday, Sept. 5, 2005. (AP)