Israel says a deal to upgrade its fleet of US-made Apache attack helicopters is being held up by Washington's refusal to release secret computer software codes.

US officials have expressed fears that giving Israel the software would allow its close ally to sell valuable technology on to third parties.

Israel has agreed to buy a dozen AH-64 Apache Longbows, a modernised version of the attack helicopter with more sophisticated targetting and radar systems. It wants an option for 12 more.

"It's obvious we need this information," Air Force Chief Major General Eitan Ben-Eliahu told reporters visiting Israel's Ramon air force base.

"It is a matter of a little bit of negotiating and explaining our needs."

Current threat

Several of Israel's existing fleet of Apaches were in action over southern Lebanon on Wednesday, where they buzzed the cities of Tyre and Sidon, causing terrified residents to shelter as they dropped flares to counter possible attack by heat-seeking missiles.

Five days of air strikes have followed successful guerrilla attacks on Israeli forces patrolling the zone Israel occupies in south Lebanon.

The Longbow enhancements - including a mast mounted radar above the rotor - increases the aircraft's ability to find targets and attack them without leaving protected positions behind hills and in valleys.

The system is considered superior to the Israel's current fleet of Apache and Cobra attack helicopters.

The deal is thought to be worth around a billion dollars and it is being paid for from the nearly $2bn in annual US military aid that Washington gives to Israel.

Israeli says it wants the software so it can modify the helicopters, if necessary, to meet future threats.

Israel's shopping list

The deal has not yet been signed, General Ben-Eliahu said, because of US official objections, although press reports in Israel say the manufacturer Boeing is not opposed to handing over the software.

In conjunction with the recent revival of the long-dormant peace process with Israel's main regional rival Syria, Israeli defence chiefs are also requesting cruise missiles and Awacs-equipped early warning aircraft from the US as part of a military aid package.

"You have to have the right deterrents so everybody understands that the alternative of peace is much more reasonable," the general said.

He also expressed the hope that American reluctance to hand over the vital Apache software codes could be overcome.

"I can understand the sensitivity, but I am sure it will be resolved very soon," he said.