US President Barack Obama will meet Afghan President Hamid Karzai in Chicago Sunday, as he launches a NATO summit that will mark a "critical milestone" in ending the war, a top official said.

Pakistani President Asif Ali Zardari will also attend the summit, with a final agreement apparently close on reopening crucial US supply lines into Afghanistan, Tom Donilon, Obama's national security advisor said Thursday.

Currently no plans have been announced for a bilateral meeting between Obama and Zardari, who last met in Seoul in March to try to mend an anti-terror alliance almost fractured in November by the killing of 26 Pakistani soldiers in a US air strike.

Obama last met Karzai during a surprise trip to Kabul earlier this month when the two leaders inked a deal cementing 10 years of US aid for Afghanistan after NATO combat troops leave in 2014.

The pact allows US troops to stay behind to train Afghan forces and pursue the remnants of Al-Qaeda for 10 years after 2014.

Donilon said the Afghan war would be discussed in detailed sessions at the two-day summit starting on Sunday, which will draw the leaders of 61 countries to Obama's adopted hometown.

He confirmed that NATO would formally decide to begin shifting its mission next year, so that lead responsibility for combat would rest on newly trained Afghan forces and foreign troops would be in "train and advise mode."

The summit will also seek consensus on the structure and financing of the $4 billion annual cost of Afghan forces after 2014 when NATO combat troops are due to leave the country.

Donilon reported progress on securing offers from US partners to ensure the full cost does not fall on the United States, noting pledges of $110 million, $100 million and $195 million a year from Britain, Australia and Germany.

He added that leaders in Chicago would also discuss their presence in Afghanistan after 2014 and the shape of a small NATO forces expected to remain to offer training and advice for Afghan forces.

"Chicago is a critical milestone in the next step towards a responsible ending of this war, towards our achieving, very importantly, our goals in this effort in Afghanistan," Donilon said.