India will finally acquire the S-400 Triumf, one of the world's most lethal surface-to-air missiles, from Russia amid a threat of tough sanctions from the US. The deal was signed between Prime Minister Narendra Modi and Russian President Vladimir Putin this afternoon, sources said. The bilateral talks between the two leaders are going on. Mr Putin is India on a two-day visit; he arrived in the national capital on Thursday. The US has hinted at tough sanctions against any nation that buys advanced military hardware from Russia. Russia has been an important supplier of defence hardware to India for decades.