Russia has completed the delivery of an S-300 surface-to-air missile system to the Syrian military as part of new security measures following the downing of a Russian plane last month.

Russian Defence Minister Sergi Shoigu on Tuesday said at a meeting of the country’s security council that its army had “finished the deliveries of S-300 systems” including a total of four launch platforms.

Shoigu said it would take three months to train the Syrian military to operate the new air defence systems, while the integration of Russian and Syrian air defence assets into a single automated system will be completed by October 20.

“We now control a close zone of up to 50km (30 miles) and the far zone of 200km (124 miles) from where attacks on the Syrian territory were launched,” Shoigu said.

Russia, a key ally of Syrian President Bashar al-Assad, joined the seven-year war in 2015.

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The decision to deliver the modern defence system was reached following the shooting down of a Russian Il-20 reconnaissance aircraft, which was accidentally hit by a Syrian missile, killing 15 troops.

Moscow said Israel was culpable as the Il-20 had caught in the crossfire as four Israeli F-16 jets attacked targets in northwestern Syria.

The Israeli military said Syria’s indiscriminate air defence fire was the cause of the incident.

‘A serious escalation’

The United States has called the Russian move to arm Syrian military “a serious escalation”.

US State Department spokeswoman Heather Nauert on Tuesday said she could not confirm that Russia had delivered the system to Syria, but said it would be a “concern” if it had.

“I hope that they did not,” Nauert told reporters in Washington, DC, when asked about the Russian announcement. “That would be sort of a serious escalation.”

Russia-Israel tightrope

Israel has, in the past six years, carried out hundreds of attacks inside Syria, varying from firing rockets to air raids, which have increased in intensity and frequency since 2012, when Iranian paramilitary fighters joined the conflict.

Israel has carried out about 200 air raids in the last two years, according to its officials.

Since it joined the Syrian conflict, Russia has generally turned a blind eye to Israeli attacks inside the country.

Israel and Russia have also maintained a hotline to prevent their air forces from clashing in the skies over Syria.

Moscow has tried to maintain good relations with Iran, which is allied with the Syrian government, as well as Israel.

A dispute between Israel and Russia could restrict Israel’s ability to mount air raids inside Syria on what it considers the greatest threat to its security from the Syria conflict: the build-up of Iranian forces or Iranian-backed Hezbollah fighters.