Russia will begin delivering S-400 surface-to-air missile systems to India by the end of 2021, state-run RIA Novosti news agency quoted a Russian official as saying on Wednesday.

India signed a $5bn deal for S-400 missiles in 2018, drawing warnings from the United States that such an acquisition would trigger sanctions as part of a wider programme against Russia.

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"The contract is being implemented on schedule. The first shipment is due by the end of 2021," Deputy Director of the Federal Service for Military-Technical Cooperation (FSMTC) Vladimir Drozhzhov said at the Defence Expo 2020 in Lucknow, India, according to RIA.

In November, the same agency cited the general director of Russian state arms exporter Rosoboronexport, Alexander Mikheev, as saying deliveries would start in September 2021.

Sergey Chemezov, the head of Russian conglomerate Rostec, said last November New Delhi has made an advance payment of $800m for the anti-aircraft missile systems, considered one of the world's best.

Moscow has long been the main supplier of military equipment to India - accounting for 62 percent of its total weapons imports during the last five years, according to the Stockholm Peace Research Institute.

In the past three years, New Delhi has bought Russian arms worth $15bn, according to Drozhzhov.

"The order portfolio has exceeded $15bn over the past three years. If we talk about the volume of contracted products, that is, since 1991, this volume has amounted to about $70bn," he said.

However, in recent years, New Delhi has inched closer to the US and Israel for weapons supply.

The US has warned that India could face sanctions if it goes ahead with the S-400 deal. It has issued similar threats to Turkey over its purchase of the missile defence system.

Under a US law, known as the Countering America's Adversaries Through Sanctions Act (CAATSA), the purchase of defence equipment from Russia can be subject to sanctions. However, the president can opt to issue a waiver.

S Samuel C Rajiv, a defence analyst and associate fellow at New Delhi's Institute for Defence Studies and Analyses (IDSA), said "the danger of sanctions" is real, but pointed to the case of Turkey, which is yet to be sanctioned by the administration of President Donald Trump.

"The sanctions' [threat] hangs on India but going forward it is difficult to predict how things will pan out - but it would be difficult for Americans to impose sanctions on India because it's going to impact the US-India defence relationship," he told Al Jazeera.

The office of India's Defence Secretary refused to comment on the report. A Defence Ministry spokesperson could not be immediately reached out for a comment.

Bilal Kuchay contributed to this report from New Delhi, India