Four U.S. soldiers were killed and three were injured Sunday when an improvised explosive device ripped through a convoy of armored Humvees driving in a remote area of southern Afghanistan, marking the deadliest attack against U.S. forces in the past two months, U.S. military officials said.

The number of U.S. troops killed in Afghanistan this year has now reached 74, the highest of any year since the U.S.-led invasion in late 2001.

The bombing in the Deh Chopan district of Zabol province was one of a series of assaults launched Sunday by insurgents across the country's south and east, even as U.S. and Afghan forces continued a major offensive to flush them from their mountain hideouts in advance of parliamentary elections scheduled for Sept. 18.

On the outskirts of Kabul, the capital, two U.S. Embassy staff members on "routine business" were lightly injured when a bomb was detonated near their armored sport-utility vehicle, officials said. Lou Fintor, the embassy spokesman, said the incident was under investigation.

In southern Kandahar province, gunmen on a motorbike assassinated Maulvi Abdullah, a prominent pro-government cleric, and a colleague as they were walking to a mosque, Afghan officials said. In a separate attack in the province, in Spin Boldak near the border with Pakistan, gunmen fired on another senior cleric, but he managed to escape. And in the northeastern province of Konar, two tanker trucks carrying fuel to a U.S. military base were ambushed and set on fire, although the drivers were released unharmed, news services reported.

The violence capped one of the bloodiest weeks since this spring, when al Qaeda and Taliban insurgents launched a wave of near-daily attacks against troops, government supporters, aid workers, civilians, and more than a half-dozen religious leaders in an apparent bid to disrupt the parliamentary elections.

On Thursday, two U.S. soldiers were killed in a roadside bombing while guarding a U.S.-funded road construction project in Kandahar province. On Saturday, an American Marine and two Afghan soldiers were killed in a clash with insurgents in Konar province.

Insurgents have taken far heavier casualties, with several hundred believed killed since U.S. and Afghan forces began an offensive in the spring in the mountains along Afghanistan's eastern border and in the south.

In the attack on the convoy Sunday, a large bomb exploded beneath a line of Humvees crossing a wooden bridge. One of the vehicles was tossed into the air, killing those inside, the Associated Press reported.

U.S. military officials said three other soldiers were injured by secondary explosions when they tried to rescue those in the damaged Humvee.

In a statement Sunday, Maj. Gen. Jason Kamiya, operational commander of the U.S.-led troops in Afghanistan, asserted that "attacks such as this strengthen, not weaken, the resolve" of U.S. and Afghan forces.

"These types of attacks also demonstrate the enemy's desperation and cowardliness," he said. "The enemy knows that he is at a race with time, a race that he will inevitably lose."