The LoR is understood to be a significant step in instigating a deal during a time that tensions are flaring with Islamabad along the contested border known as the Line of Control (LoC).
A source close to the matter said: “The LoR was issued recently and now the US has to respond with a Letter of Acceptance (LoA) to take the Apache deal forward.”
New Delhi has reportedly asked for six highly capable Boeing AH-64E Apache choppers.
India’s ministry of defence is believed to have approved the purchase in August last year.
The US’ LoA could be received within six months.
After this has been received, the two sides could soon begin negotiations and form a final deal.
The powerful choppers and accompanying equipment are expected to total around £460million (Rupee 4,168 crore).
India also spent $3.1billion (£2.2billion) in 2015 on 22 Apache helicopters and 15 Chinook choppers.
The incredibly powerful haul is expected to arrive in the middle of next year.
The agreement will mark India as the 14th country to utilise the iconic helicopters.
The AH-64 Apache chopper is armed with a 30millimetre M230 chain gun and can carry a mixture of AGM-114 Hellfire missiles and Hydra 70 rocket pods.
The vehicle is capable of dominating the skies with its T700 turboshaft engines and high-mounted exhausts.
In 2016 it was reported that the powerful copter had been tested using MBDA Brimstone anti-missile armour, making it even more impenetrable than before.
Apache’s are capable of tracking up to 128 targets a minute.
India is currently locked in border battles with Pakistan – the two nations have consistently violated the ceasefire that is supposed to be in place along the LoC.
The main area of tension is in Jammu and Kashmir that has forced residents in the region to evacuate.
Meanwhile, the possibility of conflict is also mounting between New Delhi and China following a border dispute in the Doklam area.