The S-400 ''Triumf'' is the most advanced long-range air defense missile system

India needs the S-400 ''Triumf'' missile defence system and has apprised the Trump administration of its decision to acquire them from Russia, External Affairs Minister S Jaishankar has said, exuding confidence that the US would appreciate its rationale.

India announced its intention to acquire Russian-made S-400 surface-to-air missile systems in 2015. The delivery contract worth USD 5.43 billion was signed during the visit by Russian President Vladimir Putin to India last year.

Senior Trump administration officials have cautioned India that the S-400 deal could attract sanctions under the Countering America's Adversaries Through Sanctions Act (CAATSA) law that restricts defence purchases from Russia, Iran and North Korea.

Responding to a question on the S-400 missile system purchase, Mr Jaishankar, who is in Washington on an official visit, said, "India has made a decision on the S-400 and we have discussed that with the US government."

"I am reasonably convinced of the powers of my persuasion...," Mr Jaishankar said, responding to a question from a Russian journalist on the possibility of US sanctions on India under CAATSA if New Delhi goes ahead with its decision to purchase the S-400 missile defence system from Russia.

"It would be my hope that people understand why this particular transaction is important for us, so I think of your question to me is hypothetical," he said, during his appearance at the Centre for Strategic and International Studies (CSIS), a Washington DC-based top American think tank.

The S-400 ''Triumf'' is the most advanced long-range air defense missile system that went into service in Russia in 2007. It is designed to destroy aircraft, cruise and ballistic missiles, including medium-range missiles, and can also be used against ground installations.

The S-400 can engage targets at a distance of 400 km and at an altitude of up to 30 kilometres.

During Prime Minister Narendra Modi's visit to the Far East Russian port city of Vladivostok last month, India and Russia decided to start manufacturing spare parts and components for the Russian military equipment in India under transfer of technology and set up joint ventures, as the two strategic partners tried to transform their existing buyer-seller relationship into one of collaboration.

Prime Minister Modi and Russian President Putin also pledged to elevate India-Russia partnership to new heights of cooperation.

Close cooperation between Russia and India in military and military-technical areas is a key pillar of bilateral and especially privileged strategic partnership, the two leaders said.

According to a joint statement, the two countries expressed their commitment to upgrading their defence cooperation, including by fostering joint development and production of military equipment, components and spare parts, improve the after-sales service system.

Both countries agreed to take forward ongoing engagement to encourage joint manufacturing in India of spare parts, components, aggregates and other products for maintenance of Russian-origin arms and defence equipment under the Make-in-India programme through transfer of technology and setting up of joint ventures.