NATO leaders on Monday called on Pakistan to reopen a key supply route into Afghanistan "as soon as possible" after talks with Islamabad on the border crossing faltered.

In a declaration at a summit in Chicago, the alliance expressed appreciation to Russia and Central Asian governments for allowing supply convoys through their territory and said "NATO continues to work with Pakistan to reopen the ground lines of communication as soon as possible."

"The countries in the region, particularly Pakistan, have important roles in ensuring enduring peace, stability and security in Afghanistan and in facilitating the completion of the transition process," the declaration said.

"We stand ready to continue dialogue and practical cooperation with relevant regional actors in this regard," it added.

Islamabad shut its border to NATO supplies in November after a botched US air raid that left 24 Pakistani soldiers dead.

To ferry troops, food and equipment into Afghanistan, the US-led force in Afghanistan has relied on cargo flights and a more costly northern route network that passes through Russia, Central Asia and the Caucasus.

But the Pakistani routes are crucial for NATO's planned withdrawal of 130,000 troops by the end of 2014 -- including a vast fleet of vehicles and equipment.

Last year, before the Torkham gate border crossing was shut, the United States moved more than 35,000 shipping containers through Pakistan, according to the US military.

US and Pakistani officials had predicted an agreement would be reached by the time NATO gathered in Chicago, but the negotiations became deadlocked over Islamabad's demand to charge steep fees for NATO trucks heading to Afghanistan.

US officials rejected Pakistan's proposal to charge thousands of dollars per truck and have also refused so far to issue an explicit apology for the death of the Pakistani soldiers in the November air strikes.

President Asif Ali Zardari's attendance at the Chicago summit had raised expectations the route would be reopened but he did not meet President Barack Obama and a scheduled session with NATO Secretary General Anders Fogh Rasmussen was called off.

US officials could not predict when the impasse would be resolved after Zardari met US Secretary of State Hillary Clinton on Sunday.