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.

Meanwhile, a U.S. peace envoy brought a warning to the Taliban that any government that comes to power through force in Afghanistan will not be recognized internationally after a series of cities fell to the insurgent group in stunningly quick succession.

Zalmay Khalilzad, the U.S. envoy, traveled to Doha, Qatar, where the Taliban maintain a political office, to tell the group that there was no point in pursuing victory on the battlefield because a military takeover of the capital of Kabul would guarantee they would be global pariahs. He and others hope to persuade Taliban leaders to return to peace talks with the Afghan government as American and NATO forces finish their pullout from the country.

President Joe Biden has said the U.S. military mission in Afghanistan will end on August 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.

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.

After two decades of Western military mission and billions of dollars spent training and shoring up Afghan forces, many are at odds to explain why the regular forces have collapsed, fleeing the battle sometimes by the hundreds. The fighting has fallen largely to small groups of elite forces and the Afghan air force.

This report contains information from Reuters and The Associated Press.