A U.N. envoy has met Afghanistan's new interior minister who was for years was one of the world's most wanted Islamist militants and is now part of a government trying to head off a humanitarian crisis.

The meeting between Deborah Lyons, head of the U.N. mission in Afghanistan, and Sirajuddin Haqqani focused on humanitarian assistance, Suhail Shaheen, a Taliban spokesman, said in a statement on Twitter on Thursday.

"(Haqqani) stressed that U.N. personnel can conduct their work without any hurdle and deliver vital aid to the Afghan people," he said.

Afghanistan was already facing chronic poverty and drought but the situation has deteriorated since the Taliban took over last month with the disruption of aid, the departure of tens of thousands of people including government and aid workers and the collapse of much economic activity.

U.N. Secretary-General Antonio Guterres told an international aid conference this week that Afghans were facing "perhaps their most perilous hour".

The U.N. mission in Afghanistan said that in the Wednesday meeting Lyons had stressed the "absolute necessity for all U.N. and humanitarian personnel in Afghanistan to be able to work without intimidation or obstruction to deliver vital aid and conduct work for Afghan people".

The Taliban repeatedly targeted the United Nations during the two-decades-long U.S.-led military mission in Afghanistan that ended last month with the rout of the Western-backed government by the Taliban.

In one of the bloodiest incidents, Taliban militants killed five U.N. foreign staff in an attack on a guest-house in Kabul in 2009.

More recently, gunmen attacked a U.N. compound in the city of Herat in July with rocket-propelled grenades killing a guard, while protesters in the northern city of Mazar-i-Sharif in 2011 killed seven U.N. staff.

The Haqqani network, a faction within the Taliban and for years based on the border with Pakistan, was held responsible for some of the worst militant attacks in Afghanistan during the Taliban insurgency. The United States designated the group a terrorist organisation in 2012.

Haqqani, head of the eponymous network founded by his father, is one of the FBI's most wanted men with a reward of $10 million for information leading to his arrest.

U.S. officials and members of the old U.S.-backed Afghan government for years said the Haqqani network maintained ties with al Qaeda. The Taliban have promised not to let Afghanistan be used for militant attacks on other countries.