Militants opened fire on a police vehicle near the capital early Tuesday, killing two senior police officers who were teachers at a police academy, officials said.

The violence followed a botched ambush of a U.S.-led coalition convoy south of the city late Monday that left six Afghan civilians dead, including a child, said Khan Mohammed, the police chief in Logar province (search). Three civilians were wounded.

Security forces also uncovered a cache of bombs in Kabul (search) that militants were suspected of plotting to use against international peacekeepers.

It was not immediately clear whether the two attacks were coordinated, but they underscored the security threat facing the tightly guarded capital, home to thousands of foreign aid workers and diplomats, among others.

Tuesday's assault came just before dawn as militants attacked police 30 miles east of Kabul, near a key trade route linking the capital with the eastern Pakistani border, said Ghafor Khan, a police spokesman in the eastern town of Jalalabad (search).

Khan said investigators suspect the victims were targeted because they "are teaching new police recruits and are crucial to bringing peace to our country."

The fledgling police force has been hit hard in recent months in a string of ambushes that have left dozens of officers dead.

Hours earlier, rebels fired rockets at a U.S.-led coalition convoy 10 miles south of Kabul. The rockets missed their target and instead hit three civilian cars that were traveling close behind the five military Humvee vehicles, Mohammed said.

Extra security forces rushed to the area and surrounded a run-down fort where the assailants were thought to be hiding, he said.

A coalition spokeswoman, Lt. Carmen Nicely, confirmed the ambush and said it started with a roadside bomb explosion. She said no soldiers were hurt. They fled the area and returned later with reinforcements.

The bombs discovered in Kabul were found in a junkyard of old military vehicles in the northern part of the city, said Interior Ministry spokesman Yousuf Stanekzai.

The explosives were made from old anti-personnel mines, and rebels were "suspected to be planning to use them against ISAF," he said, referring to the NATO-led International Security Assistance Force, which guards the capital.

Militants fired rockets at the northern city of Fayzabad during the past two nights, wounding a local U.N. staff member and damaging a compound belonging to the government's intelligence agency, police chief Fazil Ahmad Nazari said.

Taliban-led rebels have stepped up violence in the past half-year and killed more than 1,400 people. The bloodshed has left many southern and eastern regions off limits to aid workers and raised fears for the country's fragile democracy.