KABUL, Afghanistan (AP) — Five U.S. troops were killed by roadside bombs and insurgent fire in southern and eastern Afghanistan on Tuesday, the latest casualties in a particularly bloody spell that has left 19 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.

In Tuesday's attacks, NATO said four troops were killed by a roadside bomb in eastern Afghanistan, while a fifth died in a battle with insurgents in the country's south. No other details were given and the service members were not identified by name, as is standard procedure.

The deaths bring this month's total to 55, 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.

A 20-year-old Estonian soldier was also killed by an explosion in the southern province of Helmand when a roadside bomb struck the armored personnel carrier he was driving.

The death brought the total number of Estonian soldiers killed in Afghanistan to eight. The Baltic country has some 160 soldiers in Afghanistan as part of the NATO-led force.

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 Islamist 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 in a bid to turn the tide of the nearly nine-year war.

August is always an especially bloody month because it marks a final push by insurgents to make gains before pulling back for the winter when snow covers the high mountain passes. And while roadside bombs have taken their toll in August, NATO says deaths from the devices are down considerably from last year as a result of better detection and suppression measures.

Also Tuesday, NATO also said its forces, working with Afghan army and police, had killed 19 insurgents and captured five in a major air assault on the village of Omar in the eastern province of Kunar.

Ground forces taking part in the assault that began Monday uncovered insurgent fighting positions, along with weapons caches and ammunition stockpiles inside the village, it said.

The coalition also 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.