A loud explosion rattled the Afghan capital near the U.S. Embassy early Friday, prompting American officials to order diplomatic staff to take shelter in an underground bunker.

Lt. Commander Ken MacKillop, a spokesman for international peacekeepers, said it was not clear what caused the blast, which came a day before the start of national elections.

"We are alert and investigating," he said.

There was no immediate word of any injuries or damage. Heavily-armed U.S. and Afghan troops sealed off the roads leading to the diplomatic area.

Beth Lee, a spokeswoman for the U.S. Embassy, said staff had been ordered to take cover in an underground bunker.

The blast shattered a relatively calm lead-up to Saturday's vote, at least in the capital. It was loud enough to shake windows and rouse people from their beds.

The headquarters for the 9,000-strong NATO-led International Security Assistance Force (search) is also close to the U.S. Embassy, as are the German and Pakistani missions.

It was the first apparent attack in Kabul since August 28, when a huge car bomb outside a private U.S. security firm in Kabul (search) killed 10 people -- three of them Americans. The Americans were helping train anti-narcotics police.

Taliban (search) and Al Qaeda rebels have kept up a steady stream of attacks throughout Afghanistan since campaigning for the election began Sept. 7, but they have so far failed to launch the type of high-impact assault that might derail the vote.

Afghanistan's Interior Minister Ali Ahmad Jalali said Thursday that Afghan forces had thwarted at least 20 attacks and arrested more than 100 people since the start of the campaign, but that the rebels had managed more than 60 rocket or bomb attacks during the period, most in the provinces.

He put the death toll at more than 60 -- including 15 civilians, 19 security forces and 30 suspected rebels. In addition, six Afghan troops were taken hostage.