"Through our intelligence sources we know there's a cell here in Kabul, at least one, whose primary mission is to seek coalition or international troops and hit them with suicide bombs," Col. Tom Collins, the chief U.S. spokesman, said at a news conference in Kabul.
Also Sunday, a suicide bombing killed the governor of Afghanistan's eastern Paktika province and two other people outside his home Sunday, police said.
Three others were wounded in the attack outside Gov. Abdul Hakim Taniwal's house in the provincial capital of Gardez, said provincial police chief Abdul Annan Raufi.
The attacker, with explosives attached to his body, ran towards the governor's car and detonated the bomb as he was leaving the house, Raufi said.
Also killed were a bodyguard and Taniwal's secretary, who were riding in the car, said Interior Ministry spokesman Yousef Stanezai.
Three police on duty at the house were wounded, he said.
Taliban-led militants have stepped up attacks in Afghanistan this year, including suicide bombings.
More than 100 Taliban fighters raided a government compound in western Afghanistan early Sunday, killing two policemen and setting fire to several buildings, a police official said. The attack was the first on the town.
Taliban fighters riding in pickup trucks and firing rocket-propelled grenades and AK-47 assault rifles attacked the compound in the Farah provincial town of Kalaigar at about 1 a.m., said provincial police chief Sayed Agha.
Two police were killed by the Taliban, who also burned two rooms of the compound and a health clinic in the town, said Agha.
Agha said the Taliban raid was the first in Kalaigar. Resurgent Taliban fighters have been most active in southern provinces. But attacks have started occurring in the west amid intense NATO military operations targeting Taliban in southern provinces like Kandahar and Helmand.
On Friday, a bomber in a car rammed into a U.S. military convoy near the U.S. Embassy, killing 16 people in the deadliest suicide attack since the fall of the hardline Islamic regime in late 2001.
A purported Taliban spokesman claimed responsibility for Friday's attack. There was no immediate claim of responsibility for the assassination of the Paktika governor.
The attacker, with explosives attached to his body, detonated the bomb as the governor left the house in a car, Raufi said.
The Taliban's increased efforts come as NATO and Afghan forces intensify attacks.
NATO and Afghan forces killed 94 Taliban fighters in airstrikes and ground attacks in southern Afghanistan, the military alliance said Sunday.
The 94 militants were killed in Kandahar province's Panjwayi and neighboring Zhari districts late Saturday and early Sunday and were separate to more than 40 Taliban who died in fighting in the same areas earlier Saturday, NATO spokesman Maj. Scott Lundy said.
The killings were part of a NATO-led Operation called Operation Medusa, which began Sept. 2 and has killed at least 420 insurgents, according to the alliance. Purported Taliban spokesmen have disputed the high death counts.
U.S. soldiers killed two suspected militants Sunday in a mortar barrage in eastern Afghanistan, an American soldier said.
Gunmen inside a house in a Kunar province village of Darbart fired on an American-Afghan army patrol searching the village in eastern Afghanistan's volatile Korengal Valley, said Sgt. Maj. Jimmy Carabello, 43, from Boston, Massachusetts.
Troops positioned just outside the village fired several 120 mm mortar rounds that hit the house and killed at least two insurgents, said Carabello, of the 1st Battalion, 32nd Infantry Regiment of the 10th Mountain Division, out of Fort Drum, New York.