Having the US pay for the upgrades would also free up cash that Pakistan’s Government could use to help offset rising fuel and food costs, which have contributed to an economic crisis there, the State Department officials said.

Under the original plan sent to the Congress, the administration intended to use up to $226.5 million of the aid to refurbish two of Pakistan’s P-3 maritime patrol planes, buy it new airfield navigation aids and overhaul its troubled fleet of Cobra attack helicopters. The State Department notified Congress last week that the administration had changed its mind and would apply the funds to the F-16s.

Lawmakers immediately bridled at the shift, questioning whether the counter-terrorism money could be spent more effectively. “We need to know if this is the best way to help Pakistan combat al-Qaeda and the Taliban,” Senator Patrick J Leahy, who heads the appropriations subcommittee on State Department and foreign operations, said in a statement.

In a two-page notification to the Congress, the State Department said that upgrading the avionics, targeting and radar systems of Pakistan’s older F-16s would “increase the survivability of the aircraft in a hostile environment” and make the “F-16s a more valuable counter-terrorism asset that operates safely during day and night operations.” It stated the modernised systems would also increase the accuracy of the F-16s’ support of Pakistani ground troops, lessening the risks of civilian casualties.

Many congressional officials remain unconvinced. “Using F-16s this way is like hitting a fly with a sledgehammer,” said one senior Senate Democratic aide. It remains unclear whether any lawmaker will block or postpone the financing, and risk harming relations with Pakistan any further.