S-400 is known as Russia's most advanced long-range surface-to-air missile defence system

Defence Minister Nirmala Sitharaman today said the S-400 Triumf air defence missile deal with Russia will go ahead notwithstanding the US sanctions on military transactions with Moscow.

Referring to CAATSA (Countering America's Adversaries Through Sanctions Act), under which the Trump administration has imposed sanctions on military deals with Russia, she said it is an American law and not a UN law and India has conveyed its position on the issue to the US.

"Our defence relation with Russia has endured several decades and we have conveyed about it to a US Congressional delegation which visited India recently," Ms Sitharaman told a group of reporters at her office in South Block.

She said negotiations with Russia for the S-400 missile deal have almost come to conclusion.

The defence minister said it may take two-and-a-half to four years to implement the S-400 missile deal after it is signed.

There has been mounting concerns in India over the US sanctions against Russian defence majors including Rosoboronexport as billions of dollars of military purchases may be impacted because of the punitive measure.

The US had announced sanctions against Russia under the stringent law for its alleged meddling in the American presidential election in 2016.

CAATSA mandates the Donald Trump administration to punish entities engaging in significant transaction with the defence or intelligence establishment of Russia.

India wants to procure the long-range missile systems to tighten its air defence mechanism, particularly along the nearly 4,000-km-long Sino-India border.

In 2016, India and Russia had signed an agreement on the 'Triumf' interceptor-based missile system which can destroy incoming hostile aircraft, missiles and even drones at ranges of up to 400 km. S-400 is known as Russia's most advanced long-range surface-to-air missile defence system.

China was the first foreign buyer to seal a government-to-government deal with Russia in 2014 to procure the lethal missile system and Moscow has already started delivery of unknown number of the S-400 missile systems to Beijing.

The S-400 is an upgraded version of the S-300 systems. The missile system, manufactured by Almaz-Antey, has been in service in Russia since 2007.