ISLAMABAD (Reuters) - Pakistan put its forces on high alert after someone pretending to be India's foreign minister made a phone call to President Asif Ali Zardari threatening war after the Mumbai attacks began, Dawn newspaper said on Saturday.
"It's true," a diplomat with knowledge of the exchanges told Reuters when asked whether the report was correct.
Dawn said the Nov. 28 caller threatened military action unless Pakistan acted immediately against the perpetrators of the slaughter in Mumbai, launched two days earlier.
For the next 24 hours nuclear-armed Pakistan's air force was put on "highest alert" as the military watched anxiously for any sign of Indian aggression, the report said.
Tensions have been running high since India blamed Islamist militants based in Pakistan for the three-day rampage in its financial capital, which killed 171 people.
"War may not have been imminent, but it was not possible to take any chances," Dawn quoted a Pakistani official as saying.
The episode triggered intense international diplomacy, with some world leaders fearing India and Pakistan could slip into an accidental war, the newspaper said.
Dawn said the caller, posing as Indian Foreign Minister Pranab Mukherjee, also tried to telephone U.S. Secretary of State Condoleezza Rice, but due to specific checks by U.S. officials the call was not put through.
According to Dawn, Rice called Mukherjee in the middle of the night to ask why he had adopted such a threatening tone, but he assured her that he had not spoken to Zardari.
Pakistan's senior envoy to Britain confirmed that Pakistan had feared an attack.
"I got a call from some friend here in the government and he said the situation is pretty serious, Indians are threatening to go to war," High Commissioner Wajid Shamsul Hassan told the BBC in comments broadcast on World Service radio.
"I again talked to the president and the president also confirmed that he had received a call, a threatening call, from India," he added.
Pakistan and India, which both became nuclear-armed states in 1998, went to the brink of war in 2002 following a militant attack on the Indian parliament in December 2001.
Mukherjee said his discussions with Pakistani Foreign Minister Shah Mehmood Qureshi, who was in New Delhi that day, had been cordial.
There were frantic phone calls between Washington, Islamabad and New Delhi to cool the temperature and by the evening of Nov. 29, calm had been restored.
Dawn said Zardari's staff had bypassed usual verification checks for a call for the president, but the government said Zardari received the call after it was properly verified that it originated from India's Ministry of External Affairs.
"It is not possible for any call to come through to the president without multiple caller identity verifications," Information Minister Sherry Rehman said in a statement.
"In fact the identity of this particular call, as evident from the CLI (caller's line identification) device, showed that the call was placed from a verified official Phone Number of the Indian Ministry of External Affairs".
The newspaper said Indian officials had denied to U.S. counterparts that the call came from its ministry and said the number could have been manipulated.
It was during the hours of uncertainty that Pakistan rescinded its offer to send its Inter-Services Intelligence agency chief to India to help investigate the Mumbai attacks.
Senior Pakistani security officers referred to the aggressive tone taken by the Indian foreign minister in a briefing to journalists on Nov. 29.
They warned, in a clear message to the United States, that if Pakistan felt threatened it would move troops from the Afghan border, where they are fighting Taliban and al Qaeda, to the Indian border and abandon the war on terrorism.
Indian and U.S. officials suspect the Lashkar-e-Taiba group was behind the Mumbai attack.
The jihadi organisation, which fights Indian rule in Kashmir, has had ties with Pakistani intelligence in the past and members have forged links with al Qaeda, according to security analysts.