18 U.S. troops killed in Afghanistan since Sat.

KABUL, Afghanistan - A roadside bomb attack killed four U.S. troops in eastern Afghanistan on Tuesday, the latest casualties in a particularly bloody spell that has left 18 service members dead since Saturday.

Meanwhile, on the southern outskirts of the capital, Kabul, a gunman opened fire on a busload of Afghan Supreme Court clerks, killing three and wounding 12, the Interior Ministry reported.

Assailants on two motorcycles halted the bus Tuesday morning in the Musayi district, an area where insurgents are active, court spokesman Abdul Malik Kamawi said. One gunman then boarded the bus and opened fire with an automatic weapon, killing two people, Kamawi said. A third died later in a hospital.

"We're trying to find out who they were. For now, we can only say they are the enemies of the Afghan people," Kamawi said.

Suspicion immediately fell on Taliban insurgents who have waged a continuous campaign against Afghan government officials and institutions and have stepped up attacks in the run-up to Sept. 18 elections for the lower house of parliament. Candidates and their aides have been threatened, kidnapped and killed, and many voters say they plan to stay away from the polls for fear of violence.

No other details were given about Tuesday's bomb attacks on U.S. forces and the service members were not identified by name as is standard procedure.

The deaths bring this month's total to 54, including a Marine killed in fighting in the volatile southern province of Helmand on Friday whose death was not announced until Monday night. That is still fewer than the 66 killed in July, the deadliest month for U.S. troops in Afghanistan since the 2001 invasion. Another member of the international coalition whose nationality has not been released was killed Monday.

Almost all of the recent coalition deaths have come in southern and eastern Afghanistan where the Taliban insurgency is most deeply entrenched and where fighting has been heaviest.

Those areas are also closest to the mountainous border with Pakistan, where insurgents maintain safe havens and training bases to instruct recruits, including foreign fighters, who are later infiltrated into Afghanistan.

NATO commanders have warned casualties will mount as coalition and Afghan forces enter areas under longtime Taliban control, particularly in the hard-line Islamic movement's spiritual heartland of Kandahar province. The NATO force swelled this month to more than 140,000 - including 100,000 Americans - with the arrival of the last of the reinforcements that President Barack Obama ordered to Afghanistan in a bid to turn the tide of the nearly nine-year war.

Also Tuesday, the coalition said it killed two insurgents and wounded a third in an airstrike Monday on a Taliban commander in charge of logistics in Kandahar, including the coordination of homemade bomb attacks.

A number of Taliban and allied Haqqani Network commanders were also detained in operations Monday, including one recently returned from teaching bomb-making techniques in Pakistan, NATO said.

In Zabul province bordering Kandahar, insurgents on Monday night ambushed a convoy carrying food and other supplies, killing two private security guards and wounding five others, provincial government spokesman Mohammad Jan Rasoolyar said.