Two US chopper manufacturers, Bell and Boeing, have opted out of the Indian Air Force's tenders for attack helicopters, giving a severe blow to India's effort to get six global companies to participate in the bids.

"The US companies have refused to submit their bids for two very different reasons and have already explained their decision to the Indian defence top brass," a Defence Ministry official said in New Delhi [Images] on Wednesday.

India had released the tenders -- Request for Proposals in defence parlance -- in May this year for 22 combat helicopters to augment its fleet of around 30 Mi-25 and Mi-35 attack helicopters.

Of the six global players asked to submit their bids within three months, India received proposals from the Russian Kamov and Mil, Italian-British AgustaWestland and French Eurocopter.

Boeing, after showing initial interest in the bids, refused to submit it later, as it wanted more time to prepare a concrete proposal meeting all qualitative requirements of IAF.

It sought an eight-week extension of the deadline, originally set for August 23 this year. However, the Defence Ministry refused to provide such a long time, considering that India wanted to complete the procurement of the 22 attack helicopters before May 2011.

The government, on receiving the request from Boeing first agreed to extend the deadline by another 30 days to reset the last date for submission of the bids in September.

But Boeing, examining the RFP requirements, realised that it required more time to be able to send an adequate response and decided to withdraw from the competition, Defence Ministry officials said.

"The company studied the government's request seriously and thoroughly, and we also sent representatives for pre-bidder's conference in New Delhi with our AH-64 Apache helicopter in mind as a probable platform," Boeing officials said.

"However, after reviewing IAF's attack chopper's RFP, Boeing concluded that it will not be able to prepare a fully compliant proposal that addresses India's unique requirements with the allotted time. This was difficult, but a necessary decision to keep our standards high," they said.

Bell Helicopters, on the other hand, withdrew its participation, realising that its offering of AH-1Z Cobra attack helicopter was available for exports only through Foreign Military Sales route.

But, India had insisted in the RFP that it would procure the chopper directly from the company and not through the government route.

With the bids closing, India is now pursuing proposals received for Italian-British AgustaWestland's AW129, European consortium Eurocopter's Tiger, Russian Kamov's Ka-50 and Mil's Mi-28.