The British defense ministry says two British soldiers and two Afghan soldiers have been killed in a suspected suicide bombing in southern Afghanistan.

The ministry says the troops were caught in an explosion while on patrol in the Sangin district of Helmand province on Tuesday.

Earlier in the day, a U.S. soldier was killed in a bomb attack elsewhere in the region.

This year has been the deadliest for U.S. troops in Afghanistan since international forces toppled the Taliban in 2001.

Early Tuesday, a suicide car bomber struck Kabul's heavily secured main diplomatic and residential district Tuesday, killing at least eight people and wounding 40 others.

Authorities say the attack in the upscale neighborhood of  Wazir Akbar Khan appeared to target former Vice President Ahmad Zia Massoud. 

Massoud is the brother of Taliban opponent and famed guerrilla fighter Ahmad Shah Massoud, who was killed by al-Qaida two days before the September 11, 2001, attacks on the United States.

The former vice president survived Tuesday's blast, but two of his bodyguards were killed.  Four women were also among those killed.

The blast damaged a nearby fortified Heetal Hotel, frequented by foreigners, a guest house run by an Indian company, and several homes.  The attack took place not far from a guest house where United Nations workers were killed in a Taliban suicide and gun attack last month.

No one has claimed responsibility for Tuesday's bombing.

Afghan President Hamid Karzai condemned the bombing, calling it a "terrorist attack" against humanity and Islam.

NATO Secretary General Anders Fogh Rasmussen issued a statement, saying the attack demonstrates that militants have "no regard for the lives of innocent Afghans."  He said the Taliban will resort to any form of terrorism to stop Afghans from building a better future.