May, well before NATO Secretary General Anders Fogh Rasmussen formally extended an invitation to the President on May 15, the sources said.
The president stepped in after protracted negotiations between Pakistan and the US over the past four months failed to produce any results. The security establishment, which found itself backed into a corner after adopting a hard line against the Americans, was left with no option but to let Zardari play a role to try and end the stalemate, the sources said.
Reports in the Pakistani and Western media have contended that Pakistani officials created confusion by saying that the US should apologise for the NATO attack after Islamabad concluded a Parliamentary review of relations with Washington.
Pakistani has now apparently climbed down on the demand for an apology as Foreign Minister Hina Rabbani Khar recently said the time had come to "move on" and repair ties with the US.
The decision-making regarding ties with the US was also affected by frosty relations between Khar and Pakistan's Ambassador to the US, Sherry Rehman, who was given the status of a federal minister by Prime Minister Yousuf Raza Gilani on April 11.
At the same time, American officials have been "uncomfortable" dealing with Khar, diplomatic sources said. "It is not very easy negotiating with the Pakistani foreign minister, who doesn't show much flexibility in discussions," a source familiar with discussions between Pakistan and the US told PTI.
During talks between an American delegation led by Special Envoy Marc Grossman and a Pakistani team headed by Khar in Islamabad on April 26, the US defence and security contingent led by Peter Lavoy, Assistant Secretary for Asia and Pacific Security Affairs, left a meeting due to remarks made by Khar, sources said.
Following this development, officials in Rehman's camp leaked stories to the Pakistani media, which contended that talks with the US were being affected by a "spoiler" in the foreign office.
A report in The News even said that the handling of the talks by Khar was being "censured by the participants." Foreign office insiders acknowledged that decision-making regarding ties with the US was being affected as both Khar and Rehman had the status of minister.
It is believed that Rehman, a senior leader of the Pakistan People's Party and a close confidante of slain Premier Benazir Bhutto, had sought the status of minister to dispel the impression that she was working under Khar, who joined the PPP shortly before the last general election in 2008 after walking out of the PML-Q, which was linked to then military ruler Pervez Musharraf.