U.S. special envoy for Afghanistan and Pakistan Marc Grossman is visiting eight European nations to consult over Afghanistan before a NATO summit in Chicago charts a course forward, the State Department said on Monday.

In the lead up to the summit, "We are focused on how best to support sustainable and sufficient Afghan national security forces for Afghanistan's future and how we can further strengthen the NATO-Afghanistan Enduring Partnership," the agency said in a statement.

A series of events, including the recent killing of 16 Afghan civilians by an American soldier and the Taliban's announced suspension in talks with the United States, are complicating efforts by NATO-led coalition forces to hand over security lead to the Afghan forces and end combat mission by 2014.

When the NATO leaders meet in Chicago on May 20-21, they will finalize the transition plan as well as NATO's future support for Afghanistan and its security forces.

Grossman is touring Denmark, Norway, Sweden, Poland, the Netherlands, Germany, France and Belgium in his seven-day visit, which started on Sunday.