REUTERS/Kevin Lamarque India's defense minister told US lawmakers that New Delhi was pursuing plans to buy Russia's S-400 air-defense system. India has longstanding defense ties with Russia, and the S-400 buy is a response to practical concerns. But the US is trying to isolate Russia and expand ties with India, and the S-400 complicates things for Washington. Despite US efforts to convince other countries not to make deals with Russian defense firms, India's defense minister told US lawmakers this month that New Delhi will go ahead with its purchase of the Russian-made S-400, one of the most advanced air-defense systems on the market. "With Russia, we have had a continuous relationship of defence procurement of seven decades.