Washington: The US is working with India to provide potential alternatives to S-400 air defence systems that New Delhi has decided to purchase from Russians, a top Pentagon official said on Wednesday.

India inked an agreement with Russia in October last year to procure a batch of the missile systems at a cost of Rs 40,000 crore. India went ahead to seal the deal notwithstanding the US' warnings against it.

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

"We are very keen to see them (India) make an alternative choice. We're working with them to provide potential alternatives (to S-400)," Randall Schriver, Assistant Defence Secretary for Indo-Pacific Security Affairs told lawmakers during a Congressional hearing.

Responding to questions on the potential sanctions on India for making such a purchase from the Russians, Schriver said, "it would be an unfortunate decision" if New Delhi chose to pursue S-400 from Russia.

"Of course, we have the (CAATSA) legislation hanging over all of that," he said referring to the punitive CAATSA legislation that imposes sanctions on countries that purchase substantial military equipment from Russia.

Schriver said CAATSA legislation is not designed to be in impediment in the growing strategic partnership that the US has with India.

"It's designed to impose cost on Russia, consequence to Russia. One way or the other, we want to work through it because India is an emerging partnership for us," the top Pentagon official said.