(Adds clashes in Afghanistan)

By Jon Hemming

KABUL, June 2 (Reuters) - Taliban insurgents are fleeing south towards the Afghan border with Pakistan in the face of a U.S. Marines offensive in volatile Helmand province, the NATO commander in Afghanistan said on Monday.

U.S. Marines have been pushing south from the former Taliban stronghold of Garmsir in Helmand for a month in an operation meant to cut off insurgent infiltration routes from Pakistan.

"They have shown under some amount of pressure they flee to their sanctuaries," General Dan McNeill told a news conference.

"In the last two days we have had many reports ... that the insurgents after experiencing these several weeks of pressure below Garmsir are trying to flee to the south perhaps to go back to sanctuaries in another country," he said.

While McNeill was careful not to name any country, the only nation with which Helmand shares a border is Pakistan.

Mainly British troops have been battling the Taliban in Helmand since March 2006, capturing a string of towns in the fertile strip along the Helmand River cutting through the desert.

But Garmsir, the southermost town of any size in Helmand, and its surrounding villages had previously evaded capture.

Washington dispatched 3,200 U.S. Marines to Afghanistan in March to bolster mainly British, Canadian and Dutch troops in southern Afghanistan after other NATO allies failed to come up with reinforcements.

REGIONAL DANGER

Afghan officials have accused Pakistan of harbouring Taliban militants, giving the insurgent leadership a base from which to direct operations and allowing fighters to use Pakistani soil for training, rest and recuperation.

Pakistan admits there is a Taliban presence in its border regions beyond government control, but says it does not help the insurgents, pointing out hundreds of Pakistani troops have died fighting the militants.

NATO and Afghan officials have also cautioned Pakistan over peace talks with Pakistani Taliban insurgents, saying such truces free up the insurgents to launch more attacks into Afghanistan.

"If there are insurgencies in places not in Afghanistan, but very close by, and security forces are not taking them on, I don't think that bodes well for the whole region," said McNeill, who is to hand command of NATO's 50,000-strong International Security Assistance Force to another U.S. general on Tuesday.

"If there is no pressure on insurgents in sanctuaries out of the reach of security forces in this country then I think (insurgent) numbers are likely to grow," he said.

Still not mentioning any country by name, McNeill implied the danger of such truces was that they could backfire.

A suspected suicide car bomber killed six people and wounded 25 in the Pakistani capital on Monday.

Elsewhere in Afghanistan, Afghan forces backed by foreign troops killed 48 Taliban rebels in clashes and airstrikes in the northwest of Afghanistan on Sunday, the Interior Ministry said. (Editing by Sanjeev Miglani) ((Kabul newsroom, +93 799 335 284))

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