Islamabad/New Delhi, Dec. 6: A man posing as Pranab Mukherjee called the Pakistan President and talked in a “threatening” manner during the Mumbai attack, prompting Islamabad to put its air force on high alert, a security official and a newspaper said today.

The caller ignored the “conciliatory language of President (Asif Ali Zardari) and directly threatened… military action if Islamabad failed to immediately act against” those behind the Mumbai carnage, said a Dawn report headlined “A hoax call that could have triggered war”.

For nearly 24 hours, it said, jittery world leaders feared an accidental war as Pakistani fighter planes patrolled the skies above Islamabad and foreign minister S.M. Qureshi hurried home from India.

An intense round of international telephone diplomacy began, with US secretary of state Condoleezza Rice calling up Mukherjee and Zardari, Dawn reported. The Indian foreign minister denied any contact with the Pakistan President, leading the hoax to be uncovered, it added.

Pakistan is probing the hoax, made on November 28 night, a security official said.

In Delhi, foreign ministry officials suggested the alleged hoax call was an invention by an under-attack ISI, keen to divert attention from its “role in the Mumbai attack”.

But Pakistan information minister Sherry Rehman said Zardari had indeed received a call from a verified Indian foreign ministry number, as shown by the caller-line identification, raising speculation in Pakistan of “mischief” by an Indian official. Rehman did not reveal the nature of the call or who made it.

Dawn, however, had said the standard check on the caller’s identity was not carried out before putting him through to Zardari because of the situation during the terror attack. Rehman denied this, saying the call was processed, verified and cross-checked.

Even so, being taken in by an impostor — whoever he was — would be embarrassing for Pakistan. Hoax calls to public figures — purportedly from other public figures — are common and Queen Elizabeth II, Fidel Castro and Venezuelan President Hugo Chavez have all been victims. Recipients therefore tend to err on the side of caution.

Recently, when Barack Obama rang a US Congresswoman, she refused to believe the caller was indeed the President-elect and hung up on him twice.