"The president is leaving for India on October 4," Putin's top foreign policy aide Yury Ushakov told reporters.

"The key feature of this visit will be the signing of the agreement to deliver S-400 air defence systems," he said. "The value of the contract will be more than $5 billion."

Moscow has been negotiating to sell the S-400 long-range surface-to-air missiles to India for months.

The sale has irked India's defence partner Washington, which has wanted to wean India off Russian technology, and a senior Pentagon official said in August that sanctions against India would come under consideration if its purchase goes through.

India has signalled it will ask Washington for a special waiver from sanctions, though a US official last week said there is no guarantee it would do so.