However, it appears from his remarks on Monday that Washington has not yet decided how to deal with the transaction that requires the administration of President Joe Biden to either impose sanctions against India or give it an exemption.
"We certainly have concerns over that system, but I don't have any updates," he said in reply to a question at his daily briefing about the S400 system costing between $4 billion and $5.3 billion beginning to arrive in India.
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He said that Defence Secretary Lloyd Austin had talked about it with Indian officials when he visited India in March.
The 2017 law, Countering America's Adversaries Through Sanctions Act (CAATSA), requires the administration to sanction those buying Russian military equipment and the US has imposed sanctions on its NATO ally Turkey for buying the S400 system.
But India's emergence as a key partner of the US in the Indo-Pacific region where they confront threats from China, creates a dilemma for Washington.
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Reflecting this, Republican Senators Ted Cruz said while introducing legislation to exempt India: "Countering China's aggressive behaviour requires viable partners in Asia and beyond, and the US-Indian relationship has become a cornerstone of our multilateral efforts."
The proposed legislation introduced by him and two other Republican senators is called the "Circumspectly Reducing Unintended Consequences Impairing Alliances and Leadership Act of 2021" (CRUCIAL Act) and it seeks to exempt the Quad members from sanctions for ten years.
The other two members of the four-nation group, Japan and Australia, do not buy Russian weapons but adding them to the legislation would make it more palatable to some legislators by giving the appearance that is not specifically about India and turn the focus to China's aggressiveness, especially in the Indo-Pacific where the US has vital interests.
Democrat Senator Mark Warner, who heads the powerful Senate Intelligence Committee, along with Republican panel member John Cornyn have also written to Biden against sanctions.
But another Democrat, Bob Menendez, who heads the Senate Foreign Relations Committee, had written to Defence Secretary Lloyd Austin earlier this year that "if India chooses to go forward with its purchase of the S-400, that act will clearly constitute a significant, and, therefore, sanctionable, transaction with the Russian defence sector under Section 231 of CAATSA".
Politico reported recently that a Senate Armed Services Committee staffer "hinted" that the waiver issue would be resolved in the latest defence budget known as the National Defence Appropriation Act when it is passed.