A senior United States envoy has left for Doha and Kabul to press Taliban and Afghan government officials to begin peace talks, the US State Department has said.The US special representative for Afghanistan, Zalmay Khalilzad, left Washington on Sunday shortly after Afghan President Ashraf Ghani and his rival Abdullah Abdullah signed a power-sharing deal that could help lead to peace talks to end the country's long-running war.READ | Taliban claims Afghan bomb attack, demands prisoner releaseThe three main impediments to the start of intra-Afghan negotiations, which were to begin 10 March, have been a rise in violence, the pace of prisoner releases and the failure of Ghani and Abdullah to resolve a power struggle.While in Doha, Khalilzad will meet with Taliban officials to discuss carrying out a 29 February US-Taliban agreement that called for prisoner releases by both sides and to "press for steps necessary to commence intra-Afghan negotiations, including a significant reduction of violence", the State Department said.In Kabul, Khalilzad will meet with senior government officials "to explore steps the Afghan government needs to take to make intra-Afghan negotiations begin as soon as possible", it added.Prisoner exchangeThe key provisions of the 29 February agreement, to which the Afghan government was not a party, involved a US commitment to reduce its presence in Afghanistan to 8 600 troops by mid-July and, conditions permitting, to zero by May 2021.A prisoner exchange was also agreed under the US-Taliban deal. The Taliban wants 5 000 of its prisoners released in one go, a demand the Ghani administration has refused to meet.So far, Kabul has released about 1 000 Taliban prisoners, while the Taliban claims to have freed 263 government captives.Earlier on Monday, Taliban warned that intra-Afghan talks could not open until the prisoner swap is completed."That which is taking place in Kabul is only a repetition of the past failed experiences," Suhail Shaheen, a Taliban spokesperson, said on Twitter in what was the group's first reaction to the Ghani-Abdullah deal."Afghan sides should focus on real and sincere solution to the issue ... The prisoners' release process should be completed and the intra-Afghan negotiations should start."Last week, Ghani ordered security forces to switch to an "offensive" position against the Taliban after two deadly attacks in a day killed dozens of people.A daylight raid on a Kabul hospital on Thursday left at least 24 people dead, including mothers and infants.That attack, which triggered international outrage, was followed by a suicide bombing at a funeral which killed at least 32 mourners.Though the Taliban denied involvement in both the earlier attacks, it killed at least seven intelligence personnel in a car bomb attack in the eastern province of Ghazni on Monday.