US and Pakistani leaders Monday signaled optimism that a deal on reopening vital supply routes to NATO convoys was within reach as Islamabad ordered negotiators to seal an agreement.

President Barack Obama told reporters the United States and Pakistan were making "diligent progress" on an accord, after he briefly met Pakistani President Asif Ali Zardari on the sidelines of a NATO summit in Chicago.

Obama, however, said he had never expected the dispute to be solved during the NATO session.

Deadlock over the six-month blockade on NATO convoys on the Pakistani border has distracted from a carefully choreographed alliance meeting in Chicago, underscoring frayed relations between the West and Islamabad.

"We didn't anticipate that the supply line issue was going to be resolved by this summit. We knew that before we arrived in Chicago," Obama said.

"But we are actually making diligent progress on it," he added.

The talks on the border crossing are complicated by serious strains in US relations with Islamabad, with the US military role in Afghanistan coupled with Washington's drone air raids against militants in Pakistan's tribal areas stirring deep public resentment in Pakistan.

"We need to work through some of the tensions that have inevitably arisen after 10 years of our military presence in that region," said Obama.

Zardari, in a speech at the summit, suggested his government wanted to resolve the impasse.

Speaking to leaders from states in the NATO-led force in Afghanistan, Zardari said the cabinet's Defense Committee "decided to direct the relevant officials to conclude negotiations for resumption of the Ground Lines of Communication" needed to supply foreign troops.

His spokesman, Farhatullah Babar, later told reporters negotiators had been told to "expedite" the talks.

"However, no timeline can be given" on when a deal will be ready, Babar said.

Zardari's meetings in Chicago had produced "greater clarity" on the issue, he added.

The Pakistani ambassador to the United States, Sherry Rehman, said she hoped that differences between the two sides could be resolved "as soon as possible."

Angered over a botched US air raid that killed 24 Pakistani soldiers, Islamabad shut the supply routes in November, forcing NATO to rely on longer, more expensive northern routes through Russia and Central Asia.

The roads through Pakistan are a crucial logistical link for NATO as it plans a large-scale withdrawal of combat troops and hardware by the end of 2014.

But US officials have so far rejected Pakistani proposals to charge steep fees of several thousand dollars for each alliance truck crossing the border.

Washington has also refused to issue an explicit apology for the lethal air raid.

In his speech, Zardari called the errant American air strike "a serious setback" that "required that we review our engagement and cooperation."

British Prime Minister David Cameron acknowledged that the closed border crossing at Torkham gate was "frustrating" for the transatlantic alliance but said he expected the routes to reopen.

With the Pakistani roads shut, the northern supply routes into Afghanistan -- negotiated with Russia and a network of governments in Central Asia and the Caucasus -- have taken on growing importance.

Defense Secretary Leon Panetta on Monday met officials from five Central Asian countries on Monday -- Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan -- to thank them for allowing NATO to transport supplies over roads and railways through their territories.

"The secretary expressed his deep appreciation for their support of the Northern Distribution Network, which is key to supplying ISAF (International Security Assistance Force) forces in Afghanistan," Pentagon press secretary George Little said in a statement.

US officials acknowledge that for the planned withdrawal of NATO-led forces and equipment, the Pakistan route is much shorter and cheaper.

But analysts say Washington may try to negotiate arrangements with Russia and other states to allow weapons to be transported on the northern road and rail routes, reducing NATO's reliance on Pakistan.