Gunmen attacked the office of the United Nations (search) refugee agency Monday, throwing a grenade and firing shots but causing no injuries, on a violent day that also saw U.S. forces engage in a firefight and bombard a secret drugs laboratory.

The attack on the Kandahar (search) office shortly after 9 p.m., was just the latest in a series of assaults on the United Nations and other aid organizations that have made much of the south and east of the country off limits for development workers.

"We were sitting outside the gate when the car pulled up," said Abdul Rehman, a security guard at the office of the U.N. High Commission for Refugees in Kandahar, a former Taliban (search) stronghold. "First they threw the grenade and then opened fire."

The attackers, who were in a car, sped away when police and security staff in a nearby tent rushed to the scene and opened fire, Rehman said.

Manoel de Almeida e Silva, a U.N. spokesman in Kabul, confirmed the guard's account, though he said it was unclear if fire was returned.

Earlier Monday, police in neighboring Zabul province said suspected Taliban militants had kidnapped an Afghan aid worker.

Zabul police chief Haji Mohammed Hayub said two Afghans were shot and injured when they tried to stop the attackers from seizing the man and his car. One, hit in the face, was seriously hurt.

"They stole the vehicle and took the driver," Hayub said. "We're trying to find them."

Mullah Abdul Hakim Latifi, a Taliban spokesman, called The Associated Press to claim responsibility.

"He is in our custody. He is safe," Latifi said of the Afghan, who was a driver for the Christian emergency relief group Shelter for Life.

Zabul has been a hotbed of activity for Taliban militants, who have repeatedly targeted construction workers and aid agencies on the newly refurbished Kabul-Kandahar highway.

A Turkish engineer working on the road was abducted in October, and two Indians were kidnapped Dec. 6. All were subsequently released unharmed.

Meanwhile, in southern Uruzgan province, American and Afghan troops raided a compound, sparking a firefight in which one suspect was wounded, the U.S. military said Monday.

Troops, including special forces, came under small arms fire during the raid Sunday morning near Deh Rawood, but none was injured, spokesman Lt. Col. Bryan Hilferty said.

Uruzgan officials said the raid was in response to an attack on a U.S. convoy Saturday that injured two American soldiers. Hilferty denied such an incident, but offered no explanation for the operation.

Uruzgan governor Jan Mohammed Khan said the house belonged to Haji Ghulam Nabi, a tribal leader who Khan said had close ties to the former Taliban regime. Three of his relatives were detained, but Nabi appeared to have evaded capture, Khan said.

The U.S. military also said an A-10 ground attack aircraft Friday destroyed an illegal drugs laboratory, 60 miles northeast of Kunduz, that contained about 1.5 tons of opium, a half-ton of chemicals and processing equipment.

Provincial governor Mohammed Aman Henneini said 11 people were detained, and one was shot and injured as he tried to flee. The U.S. military had no information on the detained or injured.

Cultivation of opium poppies, the raw material for heroin, is booming in the north of Afghanistan, which already produces some three-quarters of the world's opium.

Some 11,700 soldiers from the United States and other countries are in Afghanistan hunting down suspected Taliban and Al Qaeda (search) guerillas.