KABUL, Afghanistan — 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 today.

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 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.

"People should expect significant fighting in certain areas of the south over the coming months," Collins said.

"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.