But the official, who asked not to be named, added that "although the numbers involved are relatively small, there is no set limit on what the United States is going to be willing to sell to Pakistan."
The official said that the United States was also ready to boost defence co-operation with India, including the sale of F-16s, F-18s or other aircraft, to help it develop as a major power.
"The US will respond positively to the current Indian request for information on its bid for people who are willing to sell India its next generation of multi-role combat aircraft," the official said.
He said Washington was willing to work with New Delhi in other ways, including defence co-production and technology licensing, and covering areas such as command and control, early warning systems and missile defence.
But the planned sale of the F-16s to Pakistan, which reportedly wants to buy up to 25 of the aircraft, drew an immediate expression of anger from India.
US President George W. Bush phoned Indian Prime Minister Manmohan Singh early on Friday to inform him of the plans and Singh expressed his "great disappointment at this decision," according to the Press Trust of India.
PTI quoted an Indian government spokesman as saying Singh conveyed his belief that the move could have "negative consequences for India's security environment."
"The president explained his decision to move forward on the sale of F-16s to Pakistan, as well as responding to India's request for information on the provision of multi-role combat aircraft for India," said spokeswoman Dana Perino.
A senior administration official, speaking on condition of anonymity, explained that "we will respond positively to the Indian tender for bids for combat aircraft. It's not a sale, it's not that far along in the process."
Bush, who was spending the week on his ranch near this tiny town, called Singh at 8:43 am and the two spoke until 8:52 am, according to Perino.
The two leaders also discussed US-Indian relations more broadly, including "the next steps in enhancing our expanding strategic partnership," said the spokeswoman.
Bush and Singh "discussed ways to enhance our economic energy and defence dialogue," as well as "plans for an exchange of visits over the next year," said the official.
Pakistan, which has become a major ally of the United States in its war on terror after the September 11, 2001 attacks on New York and Washington, said it would buy the latest version of the F-16.
"We welcome this good gesture and it shows good friendship between Pakistan and the United States," Information Minister Sheikh Rashid told AFP in Islamabad.
"They have offered us the latest version and it is unlimited numbers. It has pleased the people of Pakistan."
Rashid said there was no need for neighbouring India to be angry. "We want good relations with them. We have no objection if India gets anything, they can also buy the aircraft."
"This will not have lasting negative implications because the balance of power in the region continues to be tilted decisively in favour of India," Hasan Askari, a defence analyst and former head of the political science department at Punjab University in Lahore, told AFP.
The decision was announced on the same day that Pakistan said it could assist an investigation by the UN's atomic watchdog agency into whether Iran has been secretly developing nuclear weapons.
Musharraf said on Friday that Pakistan may send centrifuge parts for tests by the International Atomic Energy Agency (IAEA) to help its probe, in a turnaround from Islamabad's previously stated position.
"Any negative domestic fallout from Musharraf's willingness to co-operate with the IAEA can be countered by this good news that Pakistan will be ultimately receiving the F-16 fighter planes," said Riffat Hussain, head of the strategic studies department at Islamabad's Quaid-e-Azam University.
Pakistan, which already has F-16s in its airforce, has been seeking additional multi-role fighter jets since 1990 when a deal for 40 such planes fell through because of US concerns over the country's nuclear programme.
But relations between the United States and Pakistan warmed up after Islamabad helped the Americans topple the Taleban regime in Afghanistan in 2001.
India says that arming Pakistan with the sophisticated warplanes would upset the military balance in the region and cast a shadow over the slow dialogue process under way between the nuclear-armed neighbours.
Relations between the South Asian rivals have warmed since the peace process was launched in January last year, with both sides making a series of matching moves to boost communication links and people-to-people contacts.
The most visible sign of improvement in the relationship is their agreement last month to restart a bus service linking Azad and occupied Kashmir.