Washington: At a time when India plans to purchase five S-400 Triumf air defence systems for around $4.5 billion from Russia, the US on Saturday said its friends and allies should take into consideration the law under which any significant purchase of military equipment from Moscow would attract American sanctions.

US officials said the major defence purchase by India from Russia would attract sanctions under the Countering America's Adversaries through Sanction Act (CAATSA), which was signed into law by US President Donald Trump in August 2017 and went into effect in January this year.

Prime Minister Narendra Modi and Russian President Vladimir Putin will hold an informal summit in the Russian city of Sochi on Monday, and official sources said the possible impact of the US sanctions against Russia under CAATSA on Indo-Russia defence cooperation may also figure during the talks between the two leaders.

"CAATSA is a feature and we need to take it seriously. The (Trump) administration is always bound by US law. This is a US law. I'm hoping that not just India, but all of the partners that we engage with will understand that we will have to evaluate any potential large defence purchase from Russia seriously because that's what the law demands of us," Tina Kaidanow, Principal Deputy Assistant Secretary of State for Political-Military Affairs, told reporters during a conference call on Saturday.

Kaidanow travels to India next week, during which she will hold talks on defence trade and peacekeeping, which are among two key areas of the rapidly growing US-India partnership as envisioned in the administration's Indo-Pacific strategy.

Referring to the conversation that the US is having with India and other countries on CAATSA, Kaidanow said the US wants this to be a positive discussion, not framed on the negative.

"We are going to have to continue to have that conversation with both our Indian counterparts and others about how do we deal with the CAATSA issue. But, I will tell you again, it is US law. We need to take it seriously. Our partners need to take that into consideration as they make their decisions. I can't stress that enough," Kaidanow said.

When specifically asked if she sees the possibility of US imposing sanctions on India, if it goes ahead with its defence deals with Russia, the senior US official said that everyone should read that legislation carefully and understand its intent.

"The intent is not to sanction our partners. The intent is to emphasise how important it is that Russia's malign behaviour all over the world is countered and by virtue of purchasing large-scale Russian system, what you're doing is enabling that kind of behaviour. That's the intent of the legislation," she said.

India is not going to allow its defence engagement with Russia to be dictated by any other country, the sources said earlier this week, adding New Delhi has been lobbying with the Trump administration on the issue.

Kaidanow sought to clarify that the message was intended not just for India but for all its partners as they contemplate these purchases. She underscored the "positive incentives" to buy American products which are "good" and address relevant security needs besides making their forces interoperable in certain instances.

"Think about what you're doing when you purchase Russian product. It has a distinctly negative byproduct and that is you are creating an environment in which they are better able to do some of the things that we know are problematic," Kaidanow said.

She acknowledged the US understands the historic defence relationship between India and Russia.

"We understand all of that. It's a function of what are we talking about? Are you buying, a single truck; are you buying a large scale system ... these are things we're going to talk about," she said.