Afghanistan risks falling into regional civil war if all US and international troops leave as planned by the end of 2014, the conflict-wracked state's former interior minister warned on Thursday.

Mohammad Haneef Atmar, speaking in Washington, said a residual force of between 20,000 and 30,000 foreign soldiers was needed if fragile security gains made in recent years were to be built upon rather than lost.

"With 450,000 we have a problem at the moment," he told an audience at the Center for Strategic and International Studies think-tank in the US capital, referring to the combined number of Afghan and international forces.

"We are making progress in Helmand and Kandahar but we are not making progress in the east and southeast. Why do we believe that after more years Afghans alone will be able to manage that problem?" he said.

Atmar, who served as interior minister between 2008 and 2010, said it was overly-simplistic to assume that the Afghan insurgency will be reduced in scale after the 2014 pullout, and forecast the possibility that the government could fall.

"Why would it fall? If there is a premature drawdown of troops, if there is a significant reduction of economic assistance... and if the vacuum created is to be filled by regional actors," he said, alluding to Afghanistan's neighbors Pakistan, Iran, Russia and India.

"If these things happen we will fall. There will be a perfect scenario for a regional wargame and it will be a proxy-led civil war in Afghanistan which would lead to the disintegration of Afghanistan," he said, noting that such conditions would provide more safe-havens for insurgents.

Atmar, who was also education minister between 2006 and 2008, said the resulting security vacuum would lead the Afghan national army and police to become "factionalized" and loyalty would shift from the state to warlords.

"A significant part of Afghanistan would be controlled by the insurgents and that would provide safe haven to Al-Qaeda, Lashkar-e-Taiba and to all the other groups that do not have problems with Afghans alone," he added.

Lashkar-e-Taiba is the Pakistan-based insurgent group fighting against Indian control in Kashmir and has been blamed by India for deadly violence, most notably the November 2008 attacks in Mumbai that killed 166 people.

Afghanistan and the United States are currently negotiating a strategic partnership deal that will govern bilateral relations after NATO combat forces -- there are currently 140,000 in the war-torn state -- withdraw in 2014.