Pakistan is re-opening vital NATO supply routes into Afghanistan closed since a deadly November attack, US Secretary of State Hillary Clinton said Tuesday, adding the US was sorry for the loss of life.

During a telephone conversation with her Pakistani counterpart Foreign Minister Hina Rabbani Khar, she "informed me that the ground supply lines into Afghanistan are opening," Clinton said.

Islamabad had long said that Washington must apologize for the air raid that killed 24 Pakistani soldiers before it would re-open the routes, closed in anger after the US attack.

"Foreign Minister Khar and I acknowledged the mistakes that resulted in the loss of Pakistani military lives," Clinton said in a statement.

"We are sorry for the losses suffered by the Pakistani military. We are committed to working closely with Pakistan and Afghanistan to prevent this from ever happening again."

The blockade has forced the alliance to rely on longer, more expensive northern routes through Russia and Central Asia.

Initial hopes of a deal on re-opening the routes fell apart at a NATO summit in Chicago in May amid reports that Pakistan was demanding huge fees for each of the thousands of trucks that rumble across the border every year.

But Clinton said: "Pakistan will continue not to charge any transit fee in the larger interest of peace and security in Afghanistan and the region.

"This is a tangible demonstration of Pakistan's support for a secure, peaceful, and prosperous Afghanistan and our shared objectives in the region."