Updated

Southern Afghanistan will see "significant fighting" for several months as coalition troops push ahead with a major offensive to crush a resurgent Taliban force, the U.S. military said Wednesday.

The warning came a day after coalition and Afghan forces raided a Taliban meeting in the southern Helmand province, killing 20 insurgents, the Afghan military said. Separately, coalition soldiers accidentally fired on an unmarked police car at a checkpoint in eastern Kunar province, killing three policemen and wounding three, said Col. Tom Collins, a U.S. military spokesman.

Operation Mountain Thrust began in earnest last week with more than 10,000 Afghan, British, Canadian and American troops deploying throughout four southern provinces in the largest military operation since the Taliban regime was ousted by U.S.-led forces in 2001.

More than 600 people, mostly militants, have been killed in the past month as insurgents have launched their deadliest campaign of violence in years. At least 10 coalition soldiers have been killed in combat since mid-May.

CountryWatch: Afghanistan

"People should expect significant fighting in certain areas of the south over the coming months," Collins said during a press briefing in the capital, Kabul.

"That's the whole purpose of Mountain Thrust ... to go into the area where the government doesn't have a presence right now and take the threat out of those areas," he said.

U.S.-led soldiers and Afghan troops killed 20 insurgents Tuesday near Musa Qala, a remote town in the mountainous Helmand province, Afghan army commander Gen. Rahmatullah Roufi said. One Afghan soldier was wounded.

Coalition forces received information that insurgents had gathered there for a meeting and surrounded the area, Roufi said. Military officials recovered all 20 Taliban bodies, along with 22 AK-47s, seven rocket-propelled grenades and three heavy machine guns.

The police shootings happened when a car approached a checkpoint in Kunar province without slowing down, Collins said. Coalition soldiers saw that the occupants, who were not in uniform, were armed and "felt they were in immediate danger so they opened fire on the vehicle," he said.

Collins said the U.S.-led coalition "regrets the incident."

More than 90 enemy fighters have been killed in recent days as Taliban fighters have resisted the coalition push into areas they control, Collins said. The militants have been maintaining pressure on coalition forces with suicide attacks and ambushes.

"We are seeing the enemy operating in larger groups. They are fighting hard. They are clearly trying to stop our efforts to move into certain areas," he said.

The coalition offensive is timed to coincide with the upcoming transfer of command in the south this summer from U.S.-led forces to the NATO-led International Security Assistance Force.

"You will have significantly more troops down there as NATO progresses into the area," Collins said. "The Taliban will ultimately lose in that area."