The Pentagon has ruled out an automatic waiver for India from the punitive US sanctions over its weapons purchase from Russia, saying Washington has concerns over the nearly $5 billion missile defence system deal, ahead of the first 2+2 dialogue in New Delhi next week.

The Countering America's Adversaries Through Sanctions Act (CAATSA) waiver is intended at preventing US sanctions on countries like India.

India is planning to buy five S-400 Triumf missile air defence systems from Russia for around $4.5 billion.

US Defence Secretary Jim Mattis has publicly been a strong proponent of granting India waivers from sanctions.

Randall G Schriver, Assistant Secretary of Defence for Asian and Pacific Security Affairs, said: "I can't sit here and tell you today that a (CAATSA) waiver would necessarily be used. It would be a topic discussed at the highest level of our Government and they would make some determination".

"We understand the historical India-Russia relationship. We want to have a conversation with India not on legacy, but on future. On CAATSA, Mattis did plea for an exception for India, but I can't guarantee a waiver will be used for future purchases.

"Russia is not a country you want to have a strategic partnership," Schriver told a Washington audience at the Carnegie Endowment For International Peace yesterday.

The CAATSA was the result of Russian behaviour, not Indian, Schriver said as he discussed Defence Secretary James Mattis' priorities for the upcoming 2+2 India meeting in New Delhi.

"The Congress felt the need to take action and to demonstrate not only in words and in spirit, why we think this regime's activities are so troublesome, but to actually take concrete action to try to have a consequence and punishment for this behaviour.

"I think most people acknowledge it (CAATSA) was a flawed legislation for the reasons that some of our partners, friends and allies themselves might end up paying a price that was not intended. So Congress was very good at working with us to create this waiver opportunity," he said.

"Now we have a little more flexibility for the Secretary of State, the President to make those determinations," Schriver said, ruling out giving a guarantee that no sanctions would be imposed on India.

"I cannot sit here today and tell you if India buys X, then the waiver will be used or it won't be used.

"It would be some weighing of the concerns that the acquisition creates and how that could impact a variety of things, including the future of our defence cooperation that could put limitations on it versus wanting the political space and the ability to build out the strategic partnership with India at a more rapid pace," the Pentagon official said.

He said that S-400 is a system that's particularly troubling for a lot of reasons.

"India's a friend, it's a sovereign country. They'll make their own decisions but our preference is to seek alternatives and see if we can be a partner to India in addressing those defence needs if they choose to go down that route," he said.

The United States, he said, is willing to talk to India about meeting its defence requirements and alternatives.

"I can say we'd certainly be willing to enter into that conversation with India and we have how we can be a good partner in addressing what their real requirements are,” Schriver said.

There was an "impression that we are going to completely protect the India relationship, insulate India from any fallout from this legislation no matter what they do. 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)," he said.

US Secretary of State Mike Pompeo is travelling to New Delhi with Defence Secretary Mattis for the inaugural 2+2 ministerial dialogue on September 6.

The format of the 2+2 dialogue was agreed upon between the two sides during the visit of Prime Minister Narendra Modi to Washington in June 2017.

Earlier, External Affairs Minister Sushma Swaraj and Defence Minister Nirmala Sitharaman were to travel to Washington to take part in the meeting with their US counterparts in July, but the US postponed the dialogue citing "unavoidable reasons".