Russia and India are set to sign a deal worth more than five billion dollars on the delivery of Russian S-400 missile systems to New Delhi, according to the Kremlin.

Russian President Vladimir Putin will oversee the agreement during his trip to India this week, top Kremlin aide Yuri Ushakov said on Tuesday.

“The president is leaving for India on October 4,” Ushakov told reporters.

“The key feature of this visit will be the signing of the agreement to delive S-400 air defence systems,” he said. “The value of the contract will be more than five billion dollars.”

Moscow has been negotiating to sell the S-400 long-range surface-to-air missiles to India for months.

The sale has irked the United States, India’s defence partner, which has wanted to wean India off Russian technology.

The U.S. has imposed sanctions on Russia for its annexation of the Ukrainian peninsula of Crimea, which means any country that engages in defence or intelligence sharing with Russia could also be subject to sanctions.

A senior Pentagon official said in August that sanctions against India would come under consideration if its purchase goes through.

India has signalled it will ask Washington for a special waiver from sanctions, though a U.S. official last week said there is no guarantee it would do so.

State-of-the-art system

The S-400 missile system is a state-of-the-art weapons platform with a maximum range of 400km, considered one of the best defence systems in existence.

The acquisition of the S-400 system would be the latest in a long series of Indian defence purchases, as the country has previously bought combat planes, ships and submarines from Russia.

India is not the only country buying the air defence systems from Russia.

Other countries such as China, Turkey, Saudi Arabia and Qatar have all bought or are planning to buy the anti-aircraft missile weapon.

U.S. military officials and politicians have also expressed concerns over Turkey’s intention to buy the Russian missile system.

In June, Saudi Arabia said it would consider “all necessary measures” if Qatar closes the deal with Russia.

Despite these threats, Russia has said the supply of the missiles to Qatar will continue, with Qatar’s Foreign Minister Mohammed bin Abdulrahman Al Thani saying acquiring the system is a “sovereign” decision.

Meanwhile, Russia’s defence minister said on Tuesday that the delivery of a modern S-300 system to bolster Syria’s air defence has been completed.

Russia announced last month that it would provide the S-300s a week after it blamed Israel for the accidental shooting down of its aircraft.

However, the Kremlin said the installation of S-300 was aimed at increasing safety of Russian military and “not directed at any third country”.