The United States (US) cannot guarantee India will be exempt from sanctions if it purchases weapons and defence systems from Russia, a top Pentagon official said, reports Al Jazeera.
Randall Schriver, the Pentagon's top Asia official, called into question on Wednesday the idea that the US would protect its relationship with India and that it will be insulated from any fallout if the purchase happens.
"I would say that is a bit misleading. We would still have very significant concerns if India pursued major new platforms and systems (from Russia)," Schriver said at a think-tank event, according to Reuters news agency.
The US has imposed sanctions on Russia for its annexation of the Ukrainian peninsula of Crimea, which means any country that engages in defence or intelligence sharing with Russia could also be subject to sanctions.
"I can't sit here and tell you that they would be exempt, that we would use that waiver, that will be the decision of the president if he is faced with a major new platform and capability that India has acquired from Russia," Shriver added.
The waivers would be possible according to a new US defence bill, which gives President Donald Trump the authority to exempt countries.
Despite Secretary of Defense James Mattis saying he is a strong proponent of granting waivers to India, Schriver said that Trump having the ability to apply these waivers did not automatically mean he would.
India is in the final stages of acquiring S-400 long-range surface to air missile systems from Russia, a deal worth $6bn.
The agreement is expected to be signed by Russia later this year.
The acquisition of the S-400 system would be the latest in a long series of Indian defence purchases, as the country has previously bought combat planes, ships and submarines from Russia.
Schriver stated that the US is concerned about this planned purchase, and is willing to talk to India about potential alternatives to the Russian missile system.
India is not the only country buying the state-of-the-art system from Russia.
Other countries such as China, Turkey, Saudi Arabia and Qatar have all bought or are planning to buy the anti-aircraft missile weapon.
US military officials and politicians have also expressed concerns over Turkey's intention to buy the Russian missile system. In June, Saudi Arabia said it would consider "all necessary measures" if Qatar closes the deal with Russia.
Despite these threats, Russia has said the supply of the missiles to Qatar will continue, with Qatar's Foreign Minister Mohammed bin Abdulrahman Al Thani saying acquiring the system is a "sovereign" decision.
The S-400 missile system is a state-of-the-art weapons platform with a maximum range of 400km, considered one of the best defence systems in existence.