New York: A senior US official has left open the possibility that Washington may not impose sanctions on India for buying the Russian Triumf S-400 missile defence system but require New Delhi to tighten defence technology security to prevent snooping by Moscow.

The State Department official, who addressed Turkey buying the Russian S-400 missile defence system, did not directly address India acquiring the same system when it was raised during a briefing on Thursday, but brought up security issues in cooperating with India.

The State Department approved the $1 billion sale of advanced MK-45 5 inch/62 calibre (MOD 4) navy guns to India this week even though India had paid the Russian contractor $800 million as an advance in August for five S-400 units valued at about $5.4 billion.

This and the official''s response appeared to leave room for India to get a waiver from the Countering America''s Adversaries Through Sanctions Act (CAATSA), which could be applied to India for buying equipment from Russian companies. CAATSA bans countries from buying equipment worth more than $15 million from defence Russian companies listed under it and imposes sanctions against them.

The US has not yet introduced CAATSA sanctions against Turkey, a NATO partner, but has refused to provide it with the advanced F-35 stealth combat jets.

India is not seeking such advanced armaments from the US at this time.

President Donald Trump warned Turkish President Recep Erdogan during his recent visit that the Russian deal is going to put it at risk of sanctions.

But the official said: "The timeline on CAATSA sanctions is not prescribed or absolute. There is still plenty of scope that could be applied as to where sanctions and the breadth and depth of sanctions could be imposed upon Turkey."

The official appeared to appreciate the problems India faces in moving away from Russian defence supplies because of its decades-long dependence on the Soviet Union.

The official said that when Secretary of State Mike Pompeo and another official visited New Delhi they "had a consistent line and a discourse with our Indian counterparts at our respective levels, which is we recognise how India suffered at the fall of Soviet Union" because of its dependence on it for defence equipment.

The official added, "It was catastrophic if one was serving in the Ministry of Defence in India in the early 1990s. So we get that."

The official said that India should take stringent measures to prevent defence technology leakage.

US industry and the government are interested in co-research, co-development and co-production with India in the defence sector, "but we can''t do it in a fashion that will expose us as well as our industry," the official said.

"So for India, yes, there''s opportunity, but they have got to address their protocols and their processes on protecting defence technology and procurement processes," the official.

"What we have pushed with Indians is: tighten up your procurement processes, tighten up your defence technology security processes and protocols, and then you''re putting yourselves in a much more mature space to be a tighter, closer partner," the official said.

The official added that the US didn't want its technology "exposed because some Russians walking the shop floor decide to go, walk away and put it in their handbag or knapsack and take it back to Moscow. We''re not going to allow that."

Last year India and the US signed the Communications Compatibility and Security Agreement (COMCASA) to "facilitate access to advanced defence systems and enable India to optimally utilise its existing US-origin platforms."