NEW DELHI (AP) — India has signed a $5 billion deal to buy five Russian S-400 air defense systems despite a looming threat of U.S. sanctions on countries that trade with Russia's defense and intelligence sectors.

Russian President Vladimir Putin and India's Prime Minister Narendra Modi signed the deal in New Delhi on Friday and discussed nuclear energy, space exploration and economics.

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

The U.S. did not spare China from sanctions last month for purchasing Russian S-400 surface-to-air missile systems and fighter jets.

Officials confirmed the deal was signed after Putin and Modi made no reference to it during a news conference following their talks.