KABUL, Aug 9 (Reuters) - The United States said it was up to Afghan security forces to defend the country after Taliban militants captured a sixth provincial capital on Monday, along with border towns and trade routes.

President Joe Biden has said the U.S. military mission in Afghanistan will end on Aug. 31, arguing that the Afghan people must decide their own future and that he would not consign another generation of Americans to the 20-year war.

U.S. envoy for Afghanistan Zalmay Khalilzad has left for Qatar where he will "press the Taliban to stop their military offensive and to negotiate a political settlement," the State Department said on Monday.

In talks over three days, representatives from governments and multilateral organizations will press for "a reduction of violence and ceasefire and a commitment not to recognize a government imposed by force," the State Department said.

The Taliban, fighting to reimpose strict Islamic law after their 2001 ouster, have stepped up their campaign to defeat the government as foreign forces withdraw.

On Monday, they took Aybak, capital of the northern province of Samangan.

"Right now the Taliban are fighting with Afghan forces to capture the police headquarters and compound of the provincial governor," said Ziauddin Zia, a lawmaker in Aybak.

"Several parts of the capital have fallen to the Taliban."

The insurgents took three provincial capitals over the weekend - Zaranj in the southern province of Nimroz, Sar-e-Pul, in the northern province of the same name, and Taloqan, in northeastern Takhar province.

They had already taken the northern provincial capital of Kunduz and Lashkar Gah, capital of Helmand province.

Pentagon spokesman John Kirby said the United States was deeply concerned about the trend but that Afghan security forces had the capability to fight the insurgent group.

"These are their military forces, these are their provincial capitals, their people to defend and it's really going to come down to the leadership that they're willing to exude here at this particular moment," Kirby said.

Asked what the U.S. military can do if the Afghan security forces are not putting up a fight, Kirby said: "Not much."

U.S. officials, speaking on condition of anonymity, said that while the military had warned Biden earlier this year that provincial capitals would fall with a withdrawal of troops, they were still surprised at how quickly some of them were being taken by the Taliban.

The United States carried out less than a dozen strikes over the weekend as the Taliban overran the provincial capitals, in one instance simply destroying equipment.

One official said the Afghan forces did not ask for any support as Kunduz was being overtaken.