U.S. Secretary of State Mike Pompeo will witness the signing of a U.S.-Taliban agreement on a withdrawal of American troops from Afghanistan, President Donald Trump said on Friday, heralding an agreement that could help his re-election campaign. 

The deal, part of a wider push for Afghan reconciliation and an end to the longest U.S. war, faces many obstacles, including an election feud between Afghanistan's two leading politicians. It is expected to be signed in Qatar's capital Doha on Saturday. 

The U.S.-Taliban agreement would begin a phased withdrawal of American and coalition forces. It would require the Taliban to initiate a formal dialogue with the Afghan government and other political and civil society groups on a permanent nationwide ceasefire and power sharing in post-war Afghanistan. 

U.S. forces invaded the Southwest Asian country in 2001 to topple the Taliban rulers who provided a safe haven in which al Qaeda planned the September 11, 2001, attacks on New York and the Pentagon that killed nearly 3,000 people. 

Among the many obstacles to peace are resolving a dispute between Afghan President Ashraf Ghani and his main political rival, Chief Executive Abdullah Abdullah. Both have claimed victory in the disputed September 28 presidential election. 

There is also the undecided question of who will form the Afghan team that would negotiate with the Taliban on how to bring the insurgents into the political process and, ultimately, how to share power with the movement. 

The agreement calls for a long-planned initial drawdown to 8,600 U.S. troops from some 13,000 but it ties further cuts to the Taliban keeping promises it may find challenging, such as cutting deep ties with al Qaeda and other militant groups. 

"If the Taliban and the government of Afghanistan live up to these commitments, we will have a powerful path forward to end the war in Afghanistan and bring our troops home," Trump said in a statement, saying Pompeo would "soon" witness the signing. 

"We ... urge the Afghan people to seize this opportunity for peace and a new future for their country," he added, without addressing when or where the signing would happen. 

Earlier, Pompeo told U.S. lawmakers he had seen a "significant" reduction of violence in Afghanistan over the past six days. 

If extended for a full week, the "Reduction in Violence" pact that took effect on February 22 is expected to culminate in the signing of an agreement between top U.S. and Taliban negotiators on Saturday in Doha, the Taliban's political headquarters. 

Afghan officials met Taliban members in Qatar on Friday to discuss a prisoner swap plan. 

Testifying before the House of Representatives' Foreign Affairs Committee, Pompeo said Washington is watching to see whether neighbouring Iran may seek to play a spoiler role in Afghanistan. 

"We have seen just these last six days a significant reduction in violence in Afghanistan," Pompeo said. He said there was a history of Iran "to act as a spoiler" inside Afghanistan.