NEW DELHI, Feb 17 (Reuters) - India's opposition parties have criticised U.S. President Donald Trump's offer to sell F-35 fighters to the country, citing their high costs, even as Russia has discussed producing its most advanced jets locally in line with Prime Minister Narendra Modi's goals.

The offer from both U.S. and India's long-time defence partner Russia comes at a time when the Indian Air Force's squadrons have fallen to 31 from an approved strength of 42 and it is seeking to acquire more jets to counter China, which is rapidly building its military.

After meeting Modi in Washington last week, Trump said the U.S. will increase military sales to India starting in 2025 and will eventually provide the fifth-generation F-35 fighters made by Lockheed Martin LMT.N.

India's main opposition Congress party has used Trump ally and billionaire Elon Musk's past criticism of the fighter to target Modi's government.

"The F-35, which Elon Musk has described as 'junk', why is Narendra Modi hell-bent on buying it?" asked a post on Congress's official X account this weekend, saying that the aircraft was expensive and had high operational costs.

The U.S. government estimates that an F-35 costs around $80 million.

The Indian government has not said it intends to buy the plane and India's foreign secretary told reporters last week that the U.S. offer was at a "proposal stage", adding that the acquisition process had not started.

India's defence ministry did not immediately respond to a request for comment.

The Congress post cited a November 2024 post by Musk on X in which he shared a video of a drone swarm and captioned it: "Meanwhile, some idiots are still building manned fighter jets like the F-35".

Musk later said in another X post: "Manned fighter jets are obsolete in the age of drones anyway".

Last week, Russia offered to make in India its fifth-generation Sukhoi Su-57 fighter, with locally sourced components, saying production could begin as early as this year if India agreed.

"Russia has never shied away from transferring technology," said Amit Cowshish, former financial adviser for acquisitions at the Indian defence ministry.

"The problem is not with Russia offering transfer of technology ... we will continue to deal with Russia and buy oil and maybe buy a couple of other things, but such a big (defence) deal is likely to create its own difficulties vis-à-vis (the) U.S.," Cowshish said.

(Reporting by Shivam Patel in New Delhi; Editing by David Holmes)

((Shivam.Patel@thomsonreuters.com;))