Pentagon spokesperson John Kirby indicated the US is yet to decide on how to deal with the transaction

Washington: The US has expressed its "concern" over the delivery of S-400 Triumf surface-to-air missile systems from Russia to India but it remains undecided on how to deal with the transaction, a senior Pentagon official said.

The S-400 is known as Russia's most advanced long-range surface-to-air missile defence system.

Though there is no official comment by the Indian Air Force on the deliveries, Russia's Director of the Federal Service for Military-Technical Cooperation (FSMTC) Dmitry Shugaev told Sputnik news agency last week that the deliveries of the missiles are going on as planned.

"We've been very clear with our Indian partners about our concern over this system," Pentagon spokesperson John Kirby said at a press briefing on Monday in reply to a question on to what extent the defence department is concerned about India receiving the first S-400s.

It is learnt that deliveries of some components of the missile systems have started and all key parts are yet to reach India.

Mr Kirby, however, indicated that the US is yet to decide on how to deal with the transaction between India and Russia.

"We certainly have concerns over that system, but I don't have any updates for you," he said.

US Deputy Secretary of State Wendy Sherman during a visit to India last month said any country deciding to use the S-400 missiles is "dangerous" and not in anybody's security interest.

At the same time, she hoped that the US and India would be able to resolve differences over the procurement. It is learnt that the matter is being discussed between India and the US.

In October 2018, India had signed a USD 5 billion deal with Russia to buy five units of the S-400 air defence missile systems, despite a warning from the then Trump administration that going ahead with the contract may invite US sanctions.

The Biden administration has not yet clarified whether it will impose sanctions on India under the provisions of the Countering America's Adversaries Through Sanctions Act (CAATSA) for procuring the S-400 missile systems.

The CAATSA, which was brought in 2017, provides for punitive actions against any country engaged in transactions with Russian defence and intelligence sectors.

The US has already imposed sanctions on Turkey under the CAATSA for the purchase of a batch of S-400 missile defence systems from Russia.

Following the US sanctions on Turkey over the procurement of S-400 missile systems, there were apprehensions that Washington may impose similar punitive measures on India.

Russia has been one of India's key major suppliers of arms and ammunition.

India and Russia have also reached the final phase of negotiation for a logistics support agreement and it is likely to be signed soon, it is learnt.

In October, two powerful US Senators -- Mark Warner of the Democratic Party and John Cornyn of the Republican Party -- had urged President Biden not to impose provisions of CAATSA against India for buying the S-400 missile system as it is in America's national security interest.

"We strongly encourage you to grant a CAATSA waiver to India for its planned purchase of the S-400 Triumf surface-to-air missile system. In cases where granting a waiver would advance the national security interests of the US, this waiver authority, as written into the law by Congress, allows the President additional discretion in applying sanctions," they wrote in a letter to Mr Biden.

Both Mr Warner, Chairman of the Senate Permanent Select Committee on Intelligence, and Mr Cornyn, Senate Minority Whip for the Grand Old Party, are co-chairs of the powerful Senate India caucus, the only country specific caucus in the US Senate.