BAGRAM AIR BASE, Afghanistan (CNN) # U.S. and coalition military forces have killed at least 18 rebel fighters in an ongoing firefight in southern Afghanistan close to the border with Pakistan, in what U.S. military sources are calling the largest military engagement in Afghanistan since Operation Anaconda.

There are no reports of allied casualties.

Speaking at a morning briefing, Col. Roger King, spokesman for U.S. 18th Airborne Corps, said the battle erupted before early Monday after a small patrol of U.S. Special Forces raided a walled compound 10 miles (16km) north of the border town of Spin Boldak.

One Afghan was killed in an initial gunfight and another was captured. That detainee then told U.S. troops about a larger contingent of rebel fighters, estimated at around 80, holed up in the Adi Ghar mountains just north of the town.

AH-64 Apache attack helicopters, acting on the tip-off, flew an initial sortie from Kandahar Air Base and came under attack.

Heavier firepower # B-1 bombers, AC-130 gunships and F-16 fighters # then joined the fray, Col. King said.

Some 20 2,000 pound bombs have already been dropped in the region, King explained.

The fighters are believed to be loyal to Gulbedin Hekmatyar, a former prime minister during the civil war of early 1990s.

Hekmatyar is a Islamic fundamentalist who later took refuge in Iran and now, according to Afghan and U.S. intelligence sources, has sided with remnant Taliban and al Qaeda units to fight American troops.

This is the heaviest firefight involving U.S. troops in Afghanistan since Operation Anaconda in March 2002, which pitted U.S. and coalition forces against a concentration of Taliban fighters.