A roadside bomb attack killed four U.S. troops in eastern Afghanistan on Tuesday, NATO says.

No details were given about the attack 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.

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.

On Monday, a Canadian soldier from western Newfoundland died at a military hospital in Germany, more than a week after he was injured by a roadside bomb in Afghanistan.

With files from CBC News