KABUL, Afghanistan -- A massive bomb exploded under a wooden bridge as a convoy of armored Humvees was crossing it yesterday, killing four US soldiers and wounding three others in the deadliest assault on American forces in Afghanistan in nearly two months.
The troops were part of a major offensive against militants who have vowed to subvert legislative elections on Sept. 18 -- the next step toward democracy after more than two decades of war and civil strife.
Rebels also stepped up attacks elsewhere, wounding two US Embassy staff members in a roadside bombing in the capital and killing a pro-government cleric and a colleague in the country's south.
Although the US military operation has left dozens of suspected rebels dead or captured, a number of American troops also have been killed, including 13 this month. US and Afghan officials have warned that violence may worsen ahead of the polls.
The bomb tied to the bottom of the small bridge exploded as the last of three Humvees was slowly crossing it, said Bashir Ahmad Khan, the government chief in the Daychopan district of Zabul Province.
''It was an enormous remote-controlled bomb. The American vehicle was tossed into the air and off the bridge. It's totally destroyed, as is the bridge," he said.
The three wounded troops were hit by shrapnel from secondary explosions as they tried to pull the four soldiers out of the burning Humvee, the military statement said. The three were evacuated to a nearby base and were in stable condition.
Major General Jason Kamiya, the US-led coalition's operational commander, said the blast would ''strengthen, not weaken, the resolve" of the troops to safeguard the polls.
It was the deadliest attack on US forces since June 28, when 19 service members were killed in eastern Kunar Province after a Navy SEAL team was ambushed and a helicopter shot down.
Some 187 US service members have been killed in and around Afghanistan since the start of Operation Enduring Freedom in late 2001, including 64 during a rash of insurgent attacks in the past six months that have left about 1,000 other people dead as well.
The bloodshed has led the military to rush in an airborne infantry battalion of about 700 troops on standby in Fort Bragg, N.C., boosting the number of US troops in Afghanistan to about 20,000. About 3,100 soldiers from 19 other nations also are members of the US-led coalition.
A separate NATO-led peacekeeping force also has brought in reinforcements and now numbers about 10,500.
A helicopter carrying NATO peacekeepers crashed in a western Afghan desert and another flying with it made an emergency landing Tuesday, killing 17 Spanish troops and wounding five, although investigators have so far found no evidence that the helicopters were downed by hostile fire.
The recent violence in Afghanistan has dampened some of the optimism that prevailed after the country's inaugural presidential election passed off peacefully last fall and insurgent attacks dropped off during the winter.
The roadside bomb that exploded near a convoy of US Embassy vehicles on the outskirts of Kabul lightly wounded two American staff members, embassy spokesman Lou Fintor said.
In the southern Kandahar Province, gunmen riding a motorbike killed cleric Mawlawi Abdullah. The slaying was the latest in a string of attacks on religious leaders who have openly condemned the Taliban and other extremists.
Abdullah, a senior figure in the Islamic Ulama Council, and a colleague were killed as they walked out of a mosque after praying at dawn yesterday, Interior Ministry official Dad Mohammed Rasa said. Police set up roadblocks around the area, but no one was arrested.
Two roadside bombs also exploded near police convoys in the southern provinces of Zabul and Uruzgan late Saturday, each killing two officers, officials said.