KABUL, Afghanistan — Helicopters and a bomber attacked insurgents massing in eastern Afghanistan under cover of darkness, killing an estimated 33 people, the U.S.-led coalition said today.

Reconnaissance aircraft spotted "large groupings" of insurgents armed with heavy machine-guns and rocket-propelled grenades on Monday night in Khost province, the coalition said.

"After positively identifying the militants, coalition forces engaged them with attack helicopters and a close air support bomber, killing approximately 33 militants," spokesman 1st Lt. Nathan Perry said.

Fighting between militants and security forces is intensifying in the southern half of Afghanistan. More than 2,100 people died in the violence in the first six months, according to an Associated Press tally.

An Afghan army officer said the clash in Khost began when Taliban militants attacked coalition and Afghan forces patrolling in Tani, a district on the border with Pakistan.

Col. Mohammed Israr, a battalion commander in Khost, said Afghan intelligence reports put the number of enemy dead at about 20.

Israr also said that the group of about 50 militants had crossed from Pakistan, where some Taliban and al-Qaida militants seek refuge, and retreated in that direction under heavy coalition fire.

However, Perry said the clash took place about five miles from the frontier and "did not involve Pakistan."

Most of those killed in the violence this year have been militants, who suffer heavy losses when caught in the open by coalition aircraft. However, foreign troop deaths also are rising as militants get more effective at ambushes and roadside bombings.

At least 45 international troops died in Afghanistan in June, the deadliest month since the 2001 U.S.-led invasion to oust the Taliban.