S-400 missiles
US expresses 'concern' over delivery of Russia's S-400 missile system to India
The S-400 is known as Russia's most advanced long-range surface-to-air missile defence system
'Freedom of choice is ours': India on S-400 purchase from Russia
Last year, India agreed to buy five S-400 systems for USD 5.2 billion, and Russia has said that delivery is on track.
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