A US envoy headed Wednesday to Pakistan in a new bid to ease months of severe strains between the war partners, after lawmakers in Islamabad called for steps including an end to drone strikes.

Marc Grossman, the US special envoy for Afghanistan and Pakistan, was flying late Wednesday to Islamabad to discuss the recent parliamentary review of relations with the United States, the State Department said.

"This is a bilateral consultation about how we can improve our relationship along all of the lines that have been difficult," State Department spokeswoman Victoria Nuland told reporters.

Relations between the two nations fell into freefall last year over the US raid that killed Osama bin Laden inside Pakistan and a NATO air strike near the border with Afghanistan that killed 24 Pakistani soldiers.

The troops' deaths in November prompted Pakistan to close its border to NATO -- a main form of cooperation between Washington and Islamabad since they entered their uneasy partnership following the September 11, 2001 attacks.

In guidelines approved by parliament earlier this month, Pakistan called for an unconditional US apology over the deaths along with a ban on transporting weapons through the country and an end to drone strikes.

Nuland declined to go into detail about whether Grossman would discuss all of Pakistan's demands but said: "I think he's open to working through the results of the parliamentary review with the Pakistani government."

"We had been waiting for that review to be concluded before we could fully re-engage. So this is our opportunity to do that," she said.

President Barack Obama has called the border deaths an accident and voiced regret. But he stopped short of an apology, a step that would likely trigger criticism from his domestic opponents in an election year.

While drone attacks appear to be at a lull, the United States has been enthusiastic about such unmanned attacks to kill militants deep in Pakistani territory. Pakistan says the strikes kill civilians and fuel resentment.

A number of other US officials have visited Pakistan in recent weeks in hopes of rebuilding ties, including top military commander General Martin Dempsey, aid chief Rajiv Shah and Deputy Secretary of State Tom Nides.

Separately, the United States refrained from criticizing Pakistan for testing a nuclear-capable intermediate range missile on Wednesday, less than a week after a similar launch by Pakistan's arch-rival India.

Nuland said the United States would offer "the same message that we gave at the time of the Indian test -- that we urge all nuclear-capable states to exercise restraint regarding nuclear and missile capabilities."

"What's most important is that they do seem to have taken steps to inform the Indians, and we, as you know, are quite intent on those two countries continuing to work together and improve their dialogue," she said.

India and Pakistan have fought three full-fledged wars since 1947. But Indian policymakers largely see their nuclear program as a deterrent not to Pakistan but to its giant neighbor China.