US President Donald Trump in August signed a law threatening sanctions on any country doing business with Russia's defence and intelligence sectors.

The sanctions regime affects American allies like India, which has purchased Russian military hardware and expertise for decades.

India, the world's top defence importer, had been in talks with Russia to buy S-400 long range surface-to-air missile systems when the sanctions were announced.

There had been speculation last year when the sanctions were signed into law that the $6 billion weapons deal could be scuttled.

But Sitharaman said the long-running negotiations "have reached the final stage".

"I like to make it clear here that in all our engagements with the US, we have very clearly explained how India and Russia's defence cooperation is something which has been going on for a very long time," she told reporters on Tuesday when asked about the sanctions.

"It is a time tested relationship. And India has got quite a lot of defence assets from Russia. Assets, spares, servicing -- we have a continuous relationship with Russia.

The sanctions "cannot be impacting on us on this particular characteristic of India's Russia defence co-operation".

India has increasingly turned to the United States and France for arms purchases in recent years, but is still reliant on Russia hardware and expertise to maintain its existing arsenal.