A homicide bomber rammed into a military vehicle in eastern Afghanistan on Tuesday, killing three international troops and one Afghan civilian, officials said.

The vehicle was rocked by a blast outside the capital of Kapisa province, said Lt. Commander Christopher Hall, a spokesman for NATO forces. He said three troops were killed and one wounded, but did not provide further details or the troops' nationalities.

Provincial officials said a suicide bomber in a Toyota Corolla smashed into the military convoy. One Afghan civilian was also killed in the blast, said Abdul Halim Ayar, a spokesman for Kapisa's governor. An Interior Ministry investigator near the site confirmed one dead civilian and said five others were wounded.

Witnesses said the car had clearly targeted the convoy.

"I was driving my motorbike when I saw the car with a young man with a beard and white cap," said Sayed Najibullah, a 22-year-old shopkeeper. "He waved at me to pass," and Najibullah sped up past the car and the convoy, only to hear a large explosion behind him a few minutes later.

Roadside and suicide bombings have spiked in Afghanistan in recent months. Such attacks were up 25 percent the first four months of 2009 compared with the same period last year, U.S. military officials have said. They have predicted that bomb attacks will rise 50 percent this year to 5,700 -- up from 3,800 last year.

Such attacks killed 172 coalition forces last year -- and far more Afghan civilians -- according to military figures.

Meanwhile, U.S. forces said they killed eight Taliban fighters in a clash in the southern province of Uruzgan on Monday. The coalition said two of its troops and three Afghan policemen were wounded during the clash.

They were undergoing medical treatment and were in stable condition. The troops were on patrol when Taliban fighters attacked them with small arms fire and heavy machine-guns.