Earlier this month, India reaffirmed its commitment to a 2018 deal with Russia on the purchase of S-400 air defence systems despite the threat of US penalties.

Indian Air Force (IAF) officers are set to leave for Russia later this year to learn how to operate the S-400 long-range surface-to-air missile systems, the first batch of which New Delhi is set to receive in October 2020, The Financial Express reported.

The news came as Indian Prime Minister Narendra Modi met with US President Donald Trump, whose administration has been pressuring New Delhi to abandon the deal with Moscow by warning of "serious consequences", on the sidelines of the G20 summit in Osaka, Japan.

While the two officials did not discuss the highly contentious issue of the S-400s' delivery to India, a US State Department functionary warned New Delhi against acquiring Russia-made systems earlier this month to avoid sanctions under the Countering America's Adversaries Through Sanctions Act (CAATSA).

"With respect to the S-400, we're urging all of our allies and partners, India included, to forgo transactions with Russia that risk triggering the CAATSA sanctions... This is a time we will be encouraging India to look at alternatives", the official said last week ahead of US Secretary of State Mike Pompeo’s trip to India.

Likewise, the US State Department gave India an indirect warning that going ahead with the Russia deal could compromise further arms agreements between New Delhi and Washington.

"What causes concern with the S-400 is that it effectively could limit India's ability to increase our own interoperability. That at a certain point a strategic point has to be made about partnerships and a strategic choice about what weapon systems and platforms a country is going to adopt", principal assistant secretary of state for South and Central Asia Alice Wells told a House Foreign Affairs Committee briefing in mid-June.

The stark warnings followed a statement by the Indian Foreign Ministry in early June indicating that New Delhi had no plans to scrub the S-400 deal with Moscow in the face of US penalties.

India and Russia inked a $5.43 billion deal for S-400 air defence missile systems last October, with the first batch set to be delivered in October 2020. The remaining four will join the Indian Air Force by 2023. At the time, the United States warned that in line with the 2017 CAATSA, sanctions could not be ruled out.