India on Friday signed a $5 billion deal to buy a Russian S-400 air defense system despite a looming threat of US sanctions on countries that trade with Russia’s defense and intelligence sectors.

The system is expected to be delivered by 2020, reported India Today, adding the two nations also signed deals for more space cooperation.

Russian President Vladimir Putin and India’s Prime Minister Narendra Modi signed the deal in New Delhi and discussed nuclear energy, space exploration and economics. Officials confirmed the deal was signed, after Putin and Modi made no reference to it during a news conference following their talks.

India has requested a waiver from US sanctions intended to punish Russia for its annexation of Crimea and alleged interference in the 2016 US elections.

The US did not spare China from sanctions last month, when it purchased Russian S-400 surface-to-air missile systems and fighter jets.

However, Washington is in a difficult position when it comes to New Delhi, seeking to bolster ties with India to counter China's growing assertiveness while maintaining pressure on Russia.

Washington and New Delhi announced plans last month for joint military drills in 2019, and agreed on the exchange of sensitive military information.

The US is now India's second biggest arms supplier.

But Russia is still number one and is looking to remain so as India carries out a $100-billion upgrade of its military hardware to counter potential threats from China and Pakistan.

“Russia is a time-tested friend. I am really glad some spine has finally been shown by India,” R.R. Subramanian, a Delhi-based strategic affairs analyst, told AFP.

“It's about time we [...] showed that we are not going to be pushed around by Washington.”