"There is no time line, it is clear that no one has an exit strategy, because we have a transition strategy," Carl Bildt, whose country is currently president of the European Union, told AFP.
"It is vital that Afghans have the confidence that we will stay," he said.
There are currently more than 100,000 foreign troops in Afghanistan but the emphasis of the foreign presence was shifting, Bildt said, from military action against Taliban-liked insurgents, to training Afghan security forces and helping build a civilian governance infrastructure.
"There has to be a move from a military-heavy presence to a civilian-heavy presence," he said.
"One of the big problems in Afghanistan in the last 30 to 40 years is that there have been too many exit strategies and not enough transition strategies," Bildt said before leaving Kabul after a two-day visit to Afghanistan.
The US and NATO commander in Afghanistan on Monday submitted a long-awaited review into the eight-year war, calling for a revised strategy to defeat the Taliban and reverse the "serious" situation in the country.
The United States and NATO have called for new thinking in Afghanistan to counter record numbers of Taliban attacks since the 2001 US-led invasion.
Bildt said the emphasis of the new strategy was on "civilian, political, economic resources" to build "rule of law, governance and anti-corruption mechanisms".
"These are critical to winning this war," he said, because "this is not a conflict that can be won by military means alone".
Bildt said he met the deputy commander of NATO forces in Afghanistan, General Jim Dutton, and discussed the review submitted by General Stanley McChrystal on Monday.
The Afghan government has welcomed the civilian focus in McChrystal's review, as civilian deaths and collateral damage have caused widespread anger.
Afghanistan is bogged down in controversy over presidential elections. President Hamid Karzai is leading a painstaking vote count but the polls have been clouded in allegations of massive fraud.