KABUL, Afghanistan - A U.S. Marines commander said Wednesday his troops have killed 400 insurgents in southern Afghanistan since late April.

Col. Peter Petronzio, the commander of the 24th Marine Expeditionary Unit, said the figure came from the governor of the southern Helmand province, where his troops have been deployed since late April.

Some 2,200 U.S. Marines moved into the town of Garmser in Helmand province to clean the area of insurgents.

Helmand province is the world's largest opium poppy growing region, the main ingredient in heroin. It is also the area with the highest level of insurgent activity in the country.

After months of fighting around Garmser, Petronzio said the area is not yet secure but is more stable.

"The Taliban proved they wanted to fight for Garmser and we took the fight to them,'' he told a news conference in Kabul.

Petronzio said NATO and Afghan forces are committed to completing their mission in an area that is an important gateway for insurgent fighters smuggling weapons from Pakistan. The Marines will be replaced by British troops this fall.

"If the Taliban are waiting for us to leave, they will have a very long wait,'' he said.

Meanwhile, the top NATO commander here told The Associated Press in an interview this week that rockets and mortars fired from militants in Pakistan at U.S. and Afghan border outposts in Afghanistan have spiked in the last month.

"We have seen an increase in the eastern part of Afghanistan of cross-border indirect fires coming into some of our, not only our but Afghan'' outposts, said Gen. David McKiernan, who took command of the 40-nation International Security Assistance Force mission last month.

But McKiernan said that U.S. and NATO forces have been returning fire.

"Of course our presumption is that the threat feels safer firing (from) across that border. I'm not sure that's the case, that they're any safer, because we do return those fires, coordinated with the Pak military,'' McKiernan said.

U.S. troops have fired artillery and used airstrikes to hit militants inside Pakistan. The militants use that country's lawless border areas as a base for staging their attacks against Afghan and foreign troops here.

McKiernan offered no specifics on the number of attacks coming into Afghanistan from Pakistan, but said: "There definitely has been an increase since I've been here in the last 30 days.''

The four-star general said he thinks those attacks have spiked because militant groups have the freedom in Pakistan's lawless tribal areas to move across the Afghan-Pakistan border unimpeded and resupply and recruit in Pakistan.

McKiernan said that violence has increased in recent weeks in Afghanistan because insurgents are attacking more vulnerable targets with complex ambushes and suicide bombings, and because NATO and Afghan forces are moving into new territories and meeting resistance from fighters. He also pointed to the porous borders and militant sanctuaries in Pakistan.

"There is a continuing issue of the very porous border with Pakistan and it has allowed insurgent militant groups a greater freedom of movement across that border, a greater freedom to resupply, to provide leadership, to provide manning across that that border,'' McKiernan said.

More than 2,100 people -- mostly militants -- have been killed in insurgency-related violence in Afghanistan this year. More than 8,000 people died in attacks last year, according to the United Nations, the most since the start of the 2001 U.S.-led invasion.

Last week the Pentagon announced it has extended the tour of the 2,200 Marines in Afghanistan, after insisting for months the unit would go home on time. The unit will stay an extra 30 days and go home in early November rather than October, officials said.