Washington, October 4: The US has urged its allies to forgo transactions with Russia, warning that the S-400 missile defence system that India intends to buy would be a "focus area" for it to implement punitive sanctions against a nation undertaking "significant" business deals with the Russians.

The US administration is required under a domestic law, Countering America's Adversaries through Sanctions Act or CAATSA to impose sanctions on any country that has “significant transactions” with Iran, North Korea or Russia. The Act primarily deals with sanctions on Russian interests such as its oil and gas industry, defence and security sector, and financial institutions, in the backdrop of its military intervention in Ukraine and its alleged meddling in the 2016 US Presidential elections.

"We urge all of our allies and partners to forgo transactions with Russia that would trigger sanctions under CAATSA," a State Department Spokesperson said on Wednesday when asked about India's plan to purchase multi-billion S-400 missile defense system from Russia. India, Russia to Sign Deal For S400 Missile System on Vladimir Putin's Visit to New Delhi.

"The Administration has indicated that a focus area for the implementation of CAATSA Section 231 is new or qualitative upgrades in capability - including the S-400 air and missile defense system," the spokesperson said.

According to a media report, India and Russia will sign a deal for the multi-billion-dollar S-400 surface-to-air missile system this week. Moscow has been negotiating to sell the S-400 long-range surface-to-air missiles to India for months.

The signing of the deal for Moscow's most advanced air defence system will be overseen by Russian president Vladimir Putin, who is visiting New Delhi on October 4-5 for the annual India-Russia summit, the report said quoting a top Kremlin official said.

"The President is trying to develop a relationship and change that, but we've not been successful, at least to date," US Secretary of State Mike Pompeo told NBC news in an interview.

In September, a senior administration official told reporters that purchase of major military system like multi-billion S-400 missile defense system would be considered as a "significant transaction and thus has a potential" for CAATSA sanctions.

The official emphasized that the ultimate target of these sanctions was Russia. The CAATSA sanctions in this context are not intended to undermine the defence capabilities of any particular country. "They are instead aimed at imposing costs upon Russia in response to its malign activities," the official had said.