Russia will begin the delivery of the S-400 surface-to-air missile systems to India by the end of 2021 and there will be no delay in execution of the project, a top Russian official said on Wednesday.

In October 2018, India had signed a $5 billion deal with Russia to buy five units of the S-400 air defence missile systems, notwithstanding warning from the Trump administration that going ahead with the contract may invite United States sanctions.

Last year, India made the first tranche of payment of around $800 million to Russia for the missile systems.

"The S400 deal is being implemented as per the original schedule. First delivery of the systems will start by the end of 2021. We will fulfil our delivery commitments," Deputy Director of the Federal Service for Military-Technical Cooperation (FSMTC), Vladimir Drozhzhov, told PTI on the sidelines of the DefExpo in Lucknow.

"The defence cooperation between the two countries is very robust. We are committed to boost it further," Drozhzhov said.

The 'Triumf' interceptor-based missile system can destroy incoming hostile aircraft, missiles and even drones at ranges of up to 400 km.

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

Another official said Russia has started production of the S-400 missile systems for India and all the five units will be delivered by 2025.

The US had imposed sanctions on Russia under the stringent Countering America's Adversaries Through Sanctions Act (CAATSA).

The law also provides for punitive action against countries purchasing defence hardware from Russia.  -- PTI