Four U.S. soldiers were killed in action in Afghanistan on Saturday, one of the biggest losses for American forces since the fall of the Taliban.

A brief statement from the U.S. military in Kabul said the soldiers were from the Combined Joint Special Operations Task Force, Afghanistan, suggesting that it was Special Forces troops who were killed.

A spokeswoman said she had no more details about the incident, which brought to 90 the number of fatalities suffered by the Americans in Afghanistan, 56 of them in combat.

The soldiers were killed in the southern province of Zabol, where remnants from the ousted Taliban have staged a series of attacks in recent months. More than 700 people have been killed in violence across the country since last August, most of it blamed on Taliban and al Qaeda fighters bent on disrupting landmark elections due to be held in September.

The U.S. military has conceded that an insurgency concentrated in the south and east of the country has gathered strength in the last two months.

Also Saturday, seven Afghan soldiers and four Taliban guerrillas were killed in clashes in the southern province of Helmand, local officials said. About 20 government troops were also wounded in what appeared to be four simultaneous and coordinated attacks by suspected Taliban.

Helmand, Kandahar and Zabol used to be strongholds of the hard-line Islamic Taliban before it was ousted by U.S.-led forces in late 2001.

There are 20,000 U.S.-led soldiers in Afghanistan hunting remnants of the Taliban and al Qaeda.