India had signed a contract in 2018 for delivery of S-400 system from Russia.

Russia and India have settled the issue of India's advance payment for the delivery of S-400 long-range surface-to-air missile systems, the Russian defence agency said on Thursday.

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.

"The contract for S-400s will be implemented in compliance with the accords reached and the documents signed. The issue of making an advance payment under the contract has been resolved. We are not commenting on the details," the press office of Russia's Federal Service for Military and Technical Cooperation told the state-run Tass news agency.

The S-400 deal was opposed by the US, which is threatening to impose sanctions on the states that are acquiring weapons and military hardware from Russia. Reports said the initial payment has been delayed due to US sanctions on Russia.

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.

India acquiring the S-400 missile defence system from Russia is a "problem" for the US, Admiral Philip Davidson, who is Commander of Indo-Pacific Command, told Aspen Security Forum in Colorado in July.

"India acquiring the S-400. It's a Russian air defense system from Russia. That''s a bit of a problem. But we continue dialogue. I think they intend to do that," Davidson said.

However, India has told the US that it does not intend to give up the deal for the purchase of Russian-made S-400 air defense missile systems.

In June, External Affairs Minister S Jaishankar told his American counterpart Mike Pompeo in New Delhi that India will go by its national interest while dealing with other countries, including with sanctions-hit Russia from whom New Delhi is procuring the S-400 missile defence systems worth Rs 40,000 crore.

"We have many relationships...they have a history. We will do what is in our national interest and part of that strategic partnership is the ability of each country to comprehend and appreciate the national interest of the other," Mr Jaishankar said.

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 km.

Russia has also signed the contracts for the delivery of these systems with China (the first customer) and Turkey.

Russia will take all efforts to deliver S-400 surface-to-air missile systems to India on schedule, Minister-Counsellor of the Russian Embassy in India Roman Babushkin said on Wednesday.

"The term of the contract's implementation is well known: by 2023, these systems must be delivered to India. Russia is ready to take all necessary efforts to follow the time parameters of this agreement," he said at a news conference dedicated to the upcoming Eastern Economic Forum in Vladivostok in September.

Russian President Putin and Prime Minister Narendra Modi will hold a summit-level meeting on the sidelines of the EEF.

"We proceed from the fact that the contract will be implemented in full in accordance with the interests of both countries and the accords reached. We intend to strictly comply with them," Mr Babushkin was quoted by Tass.

Responding to a question about whether Moscow and New Delhi had agreed on mutual settlements in the national currencies upon the signing of defense contracts, the diplomat noted this was quite a specific sphere.

"Both sides are interested in making the settlements maximally protected from the negative impact of external factors, such as financial and political restrictions, which are imposed unilaterally by other countries," the Russian diplomat stressed.

"Work in this area is undoubtedly underway," he added.