This is the first deal for artillery guns since the Bofors scandal in 1980s.
"India has today signed the Letter of Acceptance which formalises the contract between India and US for these guns," sources said.
The deal for 145 American ultra-light howitzers, costing about Rs 5,000 crore was recently cleared by the Cabinet Committee on Security (CCS).
The deal was inked as atwo-day meeting of the 15thIndia-US Military Cooperation Group (MCG) began here.
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The meeting commenced with the US Co-Chair Lt Gen David H Berger, Commander US Marine Corps Forces, Pacific calling on Lt Gen Satish Dua, CISC, HQ IDS.
The MCG meeting is co-chaired by Air Marshal AS Bhonsle DCIDS (Operations), HQ IDS.
A 260-member delegation from the US Defence Forces and several officers from the three Services HQ and HQ IDS representing the Indian side are attending the bilateral event.
On the issue of M777, the sources said India had sent a Letter of Request to the US government showing interest in buying the guns which will be deployed in high-altitude areas in Arunachal Pradesh and Ladakh, bordering China.
The offsets, under which BAE Systems, manufacturer of the gun, will invest about USD 200 million, will be pursued independently.
While 25 guns will come to India in a fly-away condition, the rest will be assembled at the proposed Assembly Integration and Test facility for the weapon system in India in partnership with Mahindra.
The first two howitzers will be delivered within six months of the contract being inked, while rest will be delivered at the rate of two per month.