NATO has offered Pakistani President Asif Ali an unconditional invitation to attend the alliance's meeting in Chicago next week.

The invitation came a day after Islamabad proposed re-opening supply routes to Afghanistan, shut six months ago after a U.S. drone strike mistakenly killed 24 Pakistani soldiers near the Afghan border.

 According to a spokesman, Zardari was considering the invitation, which he added was not tied to the opening of supply lines, Reuters reported.

Allies decided to invite President (Asif Ali) Zardari of Pakistan to Chicago to the meeting on Afghanistan, NATO spokeswoman Oana Lungescu said in a statement.

This meeting will underline the strong commitment of the international community to the people of Afghanistan and to its future. Pakistan has an important role to play in that future.

Islamabad closed all NATO supply routes to Afghanistan in protest against a U.S. drone strike that killed 24 Pakistani soldiers in November, further degrading the already strained ties between the nations.

Pakistani Prime Minister Yousaf Raza Gilani has previously indicated that Islamabad's relationship with the 48 countries in NATO cannot be jeopardized to protest a U.S. drone strike.

This is not a matter of one country. The issue of NATO supply is linked with 48 countries, Gilani was quoted as saying by the Pakistani media.

Talks between Pakistan and the U.S. are going on regarding the resumption of NATO supplies in the light of Parliament's recommendations, but ties with NATO-ISAF (International Security Assistance Force) and the U.S. should be settled through dialogue on an undeviating basis, he said.

Pakistan has been facing mounting pressure from top NATO and U.S. leaders for reopening the supply routes - vital for continued operations by ISAF forces in Afghanistan.

The Afghan government has also joined NATO in pressuring Pakistan to end the blockade, which has been creating problems for the 140,000 foreign troops and Afghan citizens in the war-shattered zones.

Nearly 70 percent of the NATO supplies to Afghanistan are transported via Pakistan, according to official estimates.