Defence Minister Sergei Shoigu said President Vladimir Putin ordered additional security measures after a Syrian Soviet-era S-200 missile shot down the Russian surveillance plane by mistake last week, killing 15 in an accident Moscow blames on Israel.

"This has pushed us to adopt adequate response measures directed at boosting the security of Russian troops" in Syria, Shoigu said in a televised statement.

"(Russia will) transfer the modern S-300 air defence system to the Syrian armed forces within two weeks."

The accident was the deadliest friendly fire between Syria and its key backer Russia since Moscow's game-changing 2015 military intervention on the side of President Bashar al-Assad.

Putin and Assad discussed the additional measures and delivery of the S-300 system on the phone Monday, the Kremlin said.

- Putin blames Israel -

Putin also told Israeli Prime Minister Benjamin Netanyahu that he disagreed with the Israeli version of events and pinned the blame on the Israeli military.

The Kremlin chief had taken a more conciliatory tone last week when he described the downing as the result of "tragic accidental circumstances".

"The information provided by the Israeli military... runs counter to conclusions of the Russian defence ministry," the Kremlin said of Monday's call between Putin and Netanyahu.

"The Russian side proceeds from the fact that the actions by the Israeli air force were the main reason for the tragedy," the Kremlin added.

Defence minister Shoigu said the Syrian military had already been trained to use the S-300 system, which was set to be sent over in 2013 but held up "at the request of Israel.

"In regions near Syria over the Mediterranean Sea, there will be radio-electronic suppression of satellite navigation, on-board radar systems and communication systems of military aviation attacking objects on Syrian territory," he said.

At the Pentagon, US Defense Secretary Jim Mattis slammed Moscow's move.

"Any additional weapons going in to support Assad right now keeps him in a position of threat to the region -- and the threat is refugee flows coming out of the region, it's murder of his own people," Mattis told reporters.

"Anything like this puts (Assad) in a position, basically, to be more of an obstruction to resolving and ending this fight."

Moscow says Israeli F-16 planes which struck Latakia in western Syria last Monday later used the landing Russian Il-20 surveillance plane as "cover," which resulted in the larger Il-20 being hit by a Syrian missile.

The Russian military has said that Israel's air force informed its command in Syria via the established de-confliction hotline, but only one minute before the air strikes -- and gave the wrong target location.

Because of this, Moscow claims that the Russian air force could not keep its plane safe.

- 'Fired recklessly' -

Israel regularly carries out strikes in Syria against Assad's government, its Lebanese ally Hezbollah and Iranian targets. An Israeli military delegation travelled to Moscow last week to share information about the incident.

An Israeli official said the information showed that the Russian plane was shot down because Syrian batteries had "fired recklessly, irresponsibly and unprofessionally, long after our planes were no longer there".

He said the warning time before the strike was "much longer than one minute".

Kremlin spokesman Dmitry Peskov warned earlier Monday the accident would affect relations between the two countries.

"According to information of our military experts, the reason (behind the downing) were premeditated actions by Israeli pilots which certainly cannot but harm our relations," Peskov told journalists.

Military analyst Vladimir Sotnikov said that despite putting in place the new security measures Moscow would want to avoid a direct military clash with Israel, a key US ally.

"I don't think that the decision to send an S-300 to Syria would significantly worsen ties with Israel," he said, ruling out an escalation of the Syrian conflict.

Peskov reiterated Moscow's stance that the new measures were only to boost the safety of its troops in Syria.

"Russia in this case is acting in its interests only, these actions are not directed against third countries, but towards defending our own military," he said.

A spokesperson for the Israeli army declined to comment on Moscow's S-300 delivery.