India signed a pact with Boeing Co for purchasing four maritime spy planes, say sources.

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India signed a contract on Wednesday to buy four maritime spy planes from Boeing Co for about $1 billion, defence and industry sources told news agency Reuters. India is aiming to bolster its navy as it tries to check China's presence in the Indian Ocean.Eight of these long-range P-8I aircraft have already been deployed to track submarine movements in the Indian Ocean and on Wednesday exercised an option for four more, two defence ministry officials and an industry source told Reuters."It's a follow-on order, it was signed today," a defence ministry official told Reuters, speaking on condition of anonymity as he is not authorised to make announcements on procurements.A second defence official confirmed the value of the contract at about $1 billion and said the aircraft were expected to enter service over the next three years.Amrita Dhindsa, a spokeswoman for Boeing defence, space, and security in India, said she was not in a position to say anything on the contract and referred all questions to the defence ministry. But she said the P81 was an aircraft used not only for long-range patrol, but was also equipped with Harpoon missiles for anti-submarine warfare.India has been building up its naval surveillance capabilities since China's navy expanded its reach and sent submarines, including one that's nuclear-powered which docked in Sri Lanka.The deal, signed during a visit by the US Under Secretary of Defense for Acquisition Frank Kendall, marks a further tightening of India's ties with the United States, which has emerged as a top arms supplier in recent years for India's largely Soviet-equipped military.A US embassy spokesman was not immediately available for comment.Boeing last year completed the delivery of the last of the aircraft under the previous order worth $2.1 billion, an industry source said.The Indian navy has deployed some of its P8-I aircraft to the Andaman and Nicobar islands near the Malacca Straits and two other routes into the Indian Ocean for military and commercial shipping.