U.S. Senate Majority Leader Bill Frist said Monday that the Afghan war against Taliban fighters can't be won militarily and urged support for efforts to bring "people who call themselves Taliban" into the government.

The Tennessee Republican said he learned from briefings that Taliban fighters were too numerous and had too much popular support to be defeated on the battlefield.

"You need to bring them into a more transparent type of government," Frist said during a brief visit to a U.S. and Romanian military base in the southern Taliban stronghold of Qalat. "And if that's accomplished, we'll be successful."

Afghanistan is suffering its heaviest insurgent attacks since a U.S.-led military force toppled the Taliban in late 2001 for harbouring al-Qaeda leader Osama bin Laden.

Frist said asking the Taliban to join the government was a decision to be made by Afghan President Hamid Karzai. Karzai's spokesmen were not immediately able to be reached for comment.

Senator Mel Martinez, a Republican from Florida accompanying Frist on his trip, said negotiating with the Taliban was not "out of the question" but that fighters who refused to join the political process would have to be defeated.

"A political solution is how it's all going to be solved," he said. 

Frist and Martinez flew to Qalat during a one-day stop in Afghanistan on a regional tour that includes stops in Pakistan and Iraq.

Frist said he had hoped the U.S. would be able to withdraw its forces from Afghanistan soon. But he said the 20,000 U.S. troops in the country are still needed to support the NATO alliance, which will assume direct control over most military operations here.

"We're going to need to stay here a long time," Frist said.

The senator said he was warned to expect attacks to increase. There appears to be an "unlimited flow" of Afghans and foreigners "willing to pick up arms and integrate themselves with the Taliban," he said.

He said the only way to win in places like the volatile southern part of the country is to "assimilate people who call themselves Taliban into a larger, more representative government."

"Approaching counterinsurgency by winning hearts and minds will ultimately be the answer," Frist said. "Military versus insurgency one-to-one doesn't sound like it can be won. It sounds to me … that the Taliban is everywhere."

Taliban-related violence has increased in Afghanistan in recent months, including the volatile southern Kandahar region where roughly 2,000 Canadian soldiers are taking part in a NATO-led mission. About 20,000 American troops are supporting the roughly 20,000 NATO troops.

The top U.S. military commander in Afghanistan, Lt. Gen. Karl Eikenberry, told Pentagon reporters last month that while the Taliban enemy in Afghanistan is not extremely strong, their numbers and influence have grown in some southern sections of the country.