The US special envoy to Afghanistan, Zalmay Khalilzad, travelled to Qatar on Tuesday to urge the Taliban to end their offensive after the group overran several Afghan provincial capitals, state department said in a statement. The Taliban have increased attacks across the country since May, when the US and Nato formally began the withdrawal of their forces from Afghanistan. US President Joe Biden has said the American military mission will end on August 31. "Ambassador Khalilzad will be in Doha to help formulate a joint international response to the rapidly deteriorating situation in Afghanistan," the US State Department said "The increased tempo of Taliban military engagement, resulting civilian casualties in armed conflict between the sides, and alleged human rights atrocities are of grave concern." Mr Khalilzad will "press the Taliban to stop their military offensive", it said. Several rounds of meetings are expected to take place over three days in Doha, where the Taliban have their political office, it said. The city has been hosting peace talks between the militants and the Afghan government since September. "A negotiated peace is the only path to ending the war," the department said. Mr Biden said it was up to the Afghan people to decide their future and that he would not consign another generation of Americans to a conflict that has lasted for two decades. Taliban fighters have so far seized control of six provincial capitals, as well as border towns and trade routes. On Monday, the insurgents captured Aibak in Samangan province. It is the fifth northern provincial capital the group has overrun. They have also seized control of Zaranj, the capital of Nimruz province in the south-west. Pentagon spokesman John Kirby said the US was deeply concerned about the Taliban advance, but that Afghan security forces were capable of fighting against 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," he said. Last week, US Secretary of State Antony Blinken held talks with a senior Afghan official overseeing Kabul’s stalled talks with the Taliban and repeated the importance of an “inclusive political settlement”. Mr Blinken spoke with Abdullah Abdullah, chairman of the Afghan High Council for National Reconciliation, and reaffirmed Washington’s “commitment to seek a just and durable political settlement that ends the war in Afghanistan", State Department spokesman Ned Price said. “Both leaders deplored continuing Taliban attacks, loss of innocent Afghan lives and displacement of the civilian population and underscored the widespread international condemnation of these attacks,” he said. They also discussed “ways to accelerate peace negotiations and achieve a political settlement”, Mr Price said.