A top U.S. envoy met Pakistani government and army officers Thursday in Islamabad in an effort to get the country to reopen American and NATO supply routes to Afghanistan.

Pakistan shut the supply lines in November to protest U.S air raids that killed 24 Pakistani troops along the Afghan border. It has taken the government months to navigate the delicate path of resuscitating ties with the U.S., a difficult process in a country where anti-American sentiment is rampant.

The army used the deaths to try to extract better terms from Washington, which sees Pakistan as an essential — if unreliable — ally against al-Qaida and vital to the sustainability of any peace deal with insurgents fighting in neighboring Afghanistan.

The country has demanded that Washington apologize for the border killings and halt attacks by drone aircraft against militants in northwest Pakistan. The U.S. regards the airstrikes as essential in the fight against al-Qaida and associated groups.

Earlier this month, Pakistan's parliament finally approved new guidelines for the country in its relationship with the U.S., a decision that Washington hopes will pave the way for the reopening of the supply lines.

Marc Grossman, who is Washington's envoy for Afghanistan and Pakistan, said he didn't expect to get an immediate commitment that the routes would reopen but that "the task now is to begin a conversation about how to move forward." Grossman also repeated earlier U.S. statements of regret but didn't apologize.

Washington wants the supply routes open before a May 20-21 summit of NATO leaders in Chicago.

Grossman met Pakistan's army chief Gen. Ashfaq Parvez Kayani and foreign affairs ministry officials to discuss the supply routes.

On Friday, he planned to hold talks with Pakistan and Afghanistan officials about progress in peace talks with the Afghan Taliban. There has been very little progress, and the Taliban announced in March that they were suspending talks with the U.S.

Pakistan's army, which is considered the most powerful player in the country, has its own incentive to patch up ties: getting American military aid flowing to the country again.

The U.S. has given Pakistan billions of dollars in military aid over the past decade, but flows have largely been frozen since the middle of last year after Osama bin Laden was killed in a unilateral U.S. raid targeting his house in a Pakistani garrison town.

Meanwhile, the U.S. Embassy in Pakistan issued a fresh security warning, saying on its website that its employees would be restricted from restaurants and markets in the capital, Islamabad, for the next two weeks.

It didn't say why, but the period covers the first anniversary of the May 2 U.S. raid that killed bin Laden. Militants have been known to stage attacks on or close to the anniversary of significant events.

U.S. diplomats already operate under tight security in Pakistan, which is home to an array of violent extremists.