There were apprehensions about the payment mechanism for the deal in wake of US sanctions. (File)

India's requirement for Russia's S-400 missile defence system has been clearly conveyed to the American side, including during US Secretary of Secretary Mike Pompeo's recent visit to New Delhi, the government told the Lok Sabha on Wednesday.

In a written reply to a question, Minister of State for External Affairs V Muraleedharan said a contract for the supply of the Russian S-400 Long Range Surface to Air Missile System to India was concluded in October 2018.

On whether procurement of S-400 missiles by India has been opposed and threatened with a ban, he said the US ''Countering America's Adversaries Through Sanctions Act'' (CAATSA) requires the imposition of certain sanctions on persons or entities that have knowingly engaged in a "significant transaction" with the defence or intelligence sectors of Russia.

The US Department of State will determine whether a transaction is "significant" for the purposes of section 231 of CAATSA on a case-by-case basis, Mr Muraleedharan said.

The US National Defence Authorization Act of FY 2019 contains a provision for presidential waiver for CAATSA sanctions subject to reporting and certification requirements by the administration, he said.

The government keeps a constant watch on all developments having a bearing on India's security and takes all necessary measures to safeguard it, he said.

"Our requirement for the S-400 system has been clearly conveyed to the U.S. side, including during the visit of U.S. Secretary of State to India on June 26, 2019," Mr Muraleedharan said.

The Indian government had said earlier that it will start receiving the missile systems from Russia from October next year and the deliveries will be completed by April 2023.

There were apprehensions about the payment mechanism for the deal in the wake of the US sanctions against Russia.