"That's a totally open question," said Michael O'Hanlon, an expert at the Brookings Institution.

US commanders in Afghanistan have asked for 10,000 more combat troops for what until recently was thought of as a forgotten war. US Defense Secretary Robert Gates said Wednesday he wanted to send more forces "sooner rather than later."

The push comes on the back of a "surge" strategy in Iraq that succeeded in bringing down violence dramatically over an 18 month period with the addition of five combat brigades.

But Pentagon officials acknowledge that what worked in Iraq cannot be neatly translated to a very different situation in Afghanistan, which is larger, poorer, more populous and contains some of the world's most difficult fighting terrain.

"The environment in Iraq and the environment in Afghanistan are very different. The enemy in Iraq and the enemy in Afghanistan are very different -- the terrain, the conditions," said Bryan Whitman, a Pentagon spokesman.

"There is a certain amount of planning that takes place, and appropriately takes place, for the environment, and the mission and the enemy you are going to be deployed against," he said.

Pentagon officials have not said how many additional forces can be mustered or what they will do with them, but it's clear that a top priority is to stop the flow of fighters into Afghanistan from safe havens in Pakistan.

"The border there is a really critical issue that we're going to have to solve," Admiral Michael Mullen, the chairman of the Joint Chiefs of Staff said Wednesday.

The Pakistani government, however, has resisted US entreaties to do more to control their side of the border, pulling army troops out of the region under a truce struck with militants in March.

"It's very clear that additional (US) troops will have a big impact on insurgents coming across that border," Mullen asserted Wednesday.

But others are doubtful that more US combat troops can seal an ill-defined border that runs through towering mountains and open desert.

"You cannot seal borders," British Defence Minister Des Browne said here last week.

"We could not seal 26 miles of border between north and south of Ireland with 40,000 troops. Please do not demand of Pakistan and Afghanistan that they they try to seal the many hundreds of kilometers of mountainous border between these two countries," he said.

Moreover, sending more troops into a country with a long history of resistance to outside forces may further inflame the insurgency, some experts warn.

"The past history is that a large footprint in Afghanistan has engendered a quick turn by locals," said Sam Brannen, an analyst at the Center for International and Security Studies.

"There is a historical threat there that says that Afghans don't like large occupying forces," he said.

"If this was a NATO surge, I would tell you it's exactly what the country needs but a US surge is risky," he said.

The NATO-led International Security Assistance Force has grown from 37,500 in January 2007 to 53,000 today, and there are now about 70,000 foreign troops in Afghanistan, including American forces not under ISAF.

The Afghan National Army is supposed to grow to 80,000 troops by the end of this year, but that pales in comparison to the 566,000 security force members in Iraq.

And despite the growth in the international forces, the insurgency also has spread over the past two years, gaining intensity and lethality with tactics borrowed from Iraq -- roadside bombs, suicide attacks, and ambushes.

An assault on a combat outpost Sunday that left nine US soldiers dead showed new levels of sophistication that US military officials believe has come from improved training inside Pakistan.

"I think one of the reasons you add forces now is it's hard to know what else to do," said O'Hanlon.

"You hope that it will make a meaningful difference, but you also hoped that about the previous increment of forces, which as you know has already gone up quite a bit in Afghanistan compared to earlier years.

"There is no way to know whether this 10,000 would be the magic 10,000 to get us over the hump," he said.

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