Suspected Taliban fighters ambushed a U.S. patrol in southern Afghanistan, killing one soldier, the military said Sunday, while Afghan forces seized rebel weapons in a neighboring province.

U.S. troops defused a bomb targeting a bridge, not far from where the patrol was attacked Saturday in Helmand province (search), a district police chief said.

The flurry of incidents over the weekend underscored the security threats to Afghanistan as it plans to hold general elections in September, viewed as a keystone in rebuilding the nation after a quarter-century of war. The United Nations will help conduct the elections.

Some 20,000 U.S. forces, up from 11,000 a few months ago, are in Afghanistan to hunt down supporters of the former Taliban regime and their Al Qaeda allies who have threatened to sabotage the polls.

Despite American assertions that the rebels are a fading force, attacks against the U.S.-backed Afghan government, aid workers and coalition forces continue in the south and east.

Lt. Col. Michele DeWerth, U.S. military spokeswoman in the capital, Kabul, said one American soldier was killed and two wounded in the attack on the patrol near Girishk in Helmand. The two wounded returned to duty after medical treatment.

DeWerth said one "anti-coalition" fighter was detained at the scene. Dad Mohammed Khan (search), the intelligence chief for Helmand, said that Afghan agents had arrested two suspected Taliban for the attack on Sunday.

At least 122 U.S. troops have died, including 53 killed in action, since the start of Operation Enduring Freedo (search)m in late 2001 to topple the Taliban regime for harboring Al Qaeda. About one-third of the fatalities have occurred during counterterrorism operations in countries other than Afghanistan.

The last reported U.S. death was on May 7, when a Marine was killed in a nighttime attack on a patrol in southern Uruzgan province, the elite American force's first loss to hostile fire in Afghanistan.

District police chief Bier Jan said that on Saturday evening, U.S. forces alerted by the police defused a bomb planted at a bridge over a river in Girishk, which he said terrorists had wanted to blow up.

He said six suspects, whom he did not identify, had been arrested and an investigation was under way.

Meanwhile, Afghan police arrested Sunday six suspected Taliban for arms smuggling in Panjwayi district of neighboring Kandahar province. District police chief Saadullah Khan said police seized 80 AK-47 assault rifles found inside an oil tanker, which was being escorted by three cars.

He said two of the arrested men were brothers of an adviser to Taliban supreme leader Mullah Omar.

Khan said the men had confessed they were planning to distribute the weapons to Taliban fighters, and 500 Afghan forces had been deployed to tighten security in the district.

Meanwhile, in central Afghanistan, unidentified attackers fired a rocket at an office of the aid group CARE International, damaging its roof but causing no injuries — the latest in a string of attacks on development agency and election workers.

Gen. Atikula Ludin, a commander in Logar province, south of the Kabul, blamed "enemies of Afghanistan" for the Saturday morning attack. Another rocket had been fired at the office of a government rural development project in the same district two weeks ago, he said.