NEW DELHI: The US could offer the fifth-generation F-35 fighter jet for both the air force and navy, if the $5.43-billion deal with Russia for the S-400 air defence system is dropped.The US has been increasing pressure on India over the S-400 deal that was signed in October last year, with senior Washington officials saying it would have a direct impact on any high technology cooperation in the future.India is keeping a close watch on what happens with Turkey , a NATO ally, that has already signed up for the S-400 and has been threatened by the US with sanctions and the cancellation of its contract for F-35s.ET has learnt that senior industry leaders as well as officials from the US are visiting India, even as the deadline for action against Turkey is closing in. The defence ministry, meanwhile, is expected to shortly move ahead on the acquisition of 110 fighter jets for the air force under a strategic partnership programme. The navy is also preparing technical requirements for its upcoming purchase of 57 combat aircraft.While no official request has been received from India and the F-35 has not been formally put on offer by the US, the aircraft could be pitched as the only air platform that will be equipped and upgraded to beat the S-400 air defence systems that have also been acquired by China.Like Turkey, India has stood strong on its purchase of the S-400 but it is learnt that only partial payments have been made by New Delhi, given banking sanctions that are already in place for dealing with Russian defence entities. More stringent CAATSA (Countering America’s Adversaries Through Sanctions Act) sanctions have also been threatened against nations purchasing the S-400 from Russia.While there had been an impression that the US could give India CAATSA waivers for the S-400, recent statements by Washington suggest that this would not be the case.The US stand has been that it will not allow its modern combat aircraft to be operated in an environment where the S-400 is also operational, as it would be able to map these aircraft, enabling software upgrades and modifications to improve performance.In an effort to wean India away from the S-400, the US has already offered its NASAMS II (National Advanced Surface-to-Air Missile System) for protection of the national capital region against ballistic missiles. In addition, the US has also been in talks for its advanced Terminal High Altitude Area Defense (THAAD) and Patriot Advance Capability (PAC-3) defence systems with India, though these would come at a significantly steeper cost than the S-400 system.