Islamabad – Defense Secretary Chuck Hagel cautioned Pakistan's prime minister Monday if Pakistani regional officials followed through on their threats to block NATO supply lines for the war in Afghanistan, it could jeopardize hundreds of millions of dollars in assistance to the military here, Pentagon officials said.

In response, Prime Minister Nawaz Sharif offered assurances that his government would take action to guarantee the safe passage of U.S. and alliance shipments through the Torkham border crossing between Pakistan and Afghanistan, officials said.

Hagel arrived in the Pakistani capital for meetings to shore up one of Washington's most complicated alliances, meeting with civilian and military officials in an effort to improve a relationship that has often broken down over differences on how to fight terrorist groups within Pakistan.

It was the first visit to Pakistan by a U.S. defense secretary in almost four years. The U.S. officials, who described the session with Sharif as amicable and constructive, said the talks focused equally on supply lines, counterterrorism, regional security and economic development.

Pentagon officials said Sharif expressed his nation's outrage over continued U.S. drone strikes on suspected terrorist and militant targets inside Pakistan, some of which have killed civilians.

U.S. and NATO officials have postponed some shipments through Torkham, out of fears over security for truck drivers and their cargo during protests against the drone strikes. Imran Khan, the power broker whose PTI Party governs the surrounding Pakistani province, has threatened protests to shut down the crossing.

Senior Pentagon officials, speaking on diplomatic ground rules of anonymity, said there were no concessions by either side on counterterrorism issues, but they did agree on the need to continue talks.