An S-400 surface-to-air missile launcher is displayed at the "ARMY-2019 International Military and Technical Forum" in Moscow in June 2019

NEW DELHI -- Ahead of Russian President Vladimir Putin's visit to India for an annual summit with India's Prime Minister Narendra Modi next month, Russia has begun supplying S-400 Triumf missile systems to India, giving a boost to New Delhi's air defence capabilities along its volatile borders with China and Pakistan.

The $5.43 billion deal to buy five surface-to-air missile systems for the Indian Air Force was concluded between India and Russia in 2018. The deliveries risk triggering U.S. sanctions against India under the 2017 "Countering America's Adversaries Through Sanctions Act" (CAATSA), which imposes economic penalties on countries that purchase Russian military hardware. The threat comes despite New Delhi's strong defence and strategic ties with Washington: The two countries make up half the Quad, which is widely seen as aimed at limiting China's influence in the Indo-Pacific.

"The supplies of the S-400 air defence system to India have started and are proceeding on schedule," Dmitry Shugaev, director of the Federal Service for Military-Technical Cooperation, was quoted as saying by Russia's state-run news agency Sputnik on Sunday.

Delivery of the first S-400 missile system, which can engage aerial targets including aircraft, drones, and ballistic and cruise missiles within a range of 400 km, is expected to be completed by the end of the year.

The system comprises the firing unit, the control system and the missile itself.

"The S-400 [will help] India defend itself from any aerial threat that China and Pakistan pose to it," N.C. Bipindra, a New Delhi-based defence strategist and editor of the news portal Defence. Capital, told Nikkei Asia, pointing out that Beijing already has this missile system, and that now New Delhi has "a countermeasure."

"India is matching capabilities with China in [regard to] the S-400, and has the upper hand against aerial threats from Pakistan, which does not have this missile system," Bipindra said.

India and China have been locked in a tense border standoff in eastern Ladakh along their disputed Himalayan border since May 2020. A clash in the Galwan Valley left 20 Indian soldiers dead in June last year, the first deadly clash between the nuclear-armed Asian neighbours in 45 years. In February, China acknowledged for the first time that four of its soldiers were also killed in that clash.

India's ties with Pakistan, meanwhile, have frayed over the last several years as direct talks have stalled and tensions continue over the disputed region of Kashmir.

The S-400 shipments to New Delhi come as Putin is expected to visit India next month for an annual meeting with Modi. The precise date of the summit has not been announced, but a range of agreements is expected in fields such as defence, trade and science and technology.

The two countries are also expected to hold their first "two-plus-two" dialogue ahead of the summit, featuring the foreign and defence ministers of both nations. India has established a two-plus-two mechanism only with three other countries -- the U.S., Japan and Australia. The two-plus-two meeting with Russia will be the first outside the Quad.

India, which has been buying military hardware from Russia for seven decades, is hoping to get a sanctions waiver from the U.S. for the S-400 deal.

In late October, U.S. senators and India Caucus co-chairs Mark Warner, a Virginia Democrat, and John Cornyn, a Republican from Texas, sent a letter to President Biden asking him to exempt New Delhi from the CAATSA sanctions.

"We strongly encourage you to grant a CAATSA waiver to India for its planned purchase of the S-400 Triumf surface-to-air missile system," they wrote, adding that imposing sanctions could have "a deleterious effect on a strategic partnership with India, while at the same time, not achieve the intended purpose of deterring Russian arms."

These efforts on Capitol Hill show that a key segment of U.S. policymakers and lawmakers favor such an exemption, Bipindra said.

"Considering that India is a major defence partner of the U.S., it is quite possible Washington may not impose CAATSA sanctions on India, and may provide a waiver because of the fact that India is a very important nation in the Indo-Pacific region, where China is already threatening the U.S. supremacy," he added.