The United Nations wants this weekend's NATO summit to make "strong" pledges to cover the estimated $4.1 billion a year cost for Afghan security forces once international troops leave, the UN envoy to Afghanistan said Friday.

The Chicago summit on Sunday and Monday is to concentrate on Afghanistan where the 130,000 US and other international forces are set to leave by the end of 2014.

Jan Kubis, the UN special representative for Afghanistan, told AFP the summit participants must make "strong clear predictable commitments for the years to come.

"The target of $4.1 billion is clear and countries should come as close as possible to this target," he said.

Kubis said that "political pledges or statements of goodwill" would not be enough.

The Chicago summit will be followed by a conference of international donors in Tokyo in July to raise money for non-military reconstruction. An appeal for several billion dollars is expected to be made there.

The United States, Germany and other countries have already signed bilateral deals promising aid to Afghanistan after 2014. Kubis said the international community must send the message to Afghans: "We are going to stay for quite a while with you."

The aim must be "to create conditions that enable Afghanistan to maintain a major degree of security and stability for the overall majority of its population, that is what is happening and that's what I expect for the years to come," he said.