Russia says it will supply Syria with a more modern S-300 missile defense system in the coming weeks, over the objections of Israel.
Russian defense minister Sergei Shoigu said President Vladimir Putin ordered the upgrade after Syrian air defenses mistook a Russian plane for an Israeli aircraft last week, killing all 15 Russians on board.
Russia has blamed Israel, whose military was conducting airstrikes on Syria at the time, for the incident.
Moscow said Syria's older S-200 missile defense system is not advanced enough to distinguish Russian planes as friendly.
Shoigu said Monday "this has pushed us to adopt adequate response measures directed at boosting the security of Russian troops" in Syria.
Israeli Prime Minister Benjamin Netanyahu told Putin in a telephone call Monday that supplying advanced weapon systems to "irresponsible hands will increase the dangers in the region," according to Netanyahu's office.
Putin said the Russian move was "aimed primarily at fending off any potential threat to the lives of Russian servicemen,'' according to the Kremlin.
U.S. National Security Advisor John Bolton advised Russia to reconsider the decision, saying the new missile system would be a "significant escalation" to already high tensions in the region.
Last week, Russian military spokesman Major General Igor Konashenkov said Israeli forces were carrying out an attack in Syria's Latakia province when they hid behind the Russian plane, using it to shield themselves from Syrian missiles.
Putin has since said "a chain of tragic circumstances" lead to the downing of the plane.
Israel said it had warned Russia of the airstrike in advance and said its jets were already back in Israeli airspace when Syria fired its missile.
The Russian defense ministry said Israel's warning came less than a minute before the airstrike. It accused the Israelis of using the Russian plane as a cover to avoid Syrian air defense systems.
Israel has long lobbied Moscow not to provide the S-300 to Syria, fearing the missile system would hinder its ability to strike Iranian and Hezbollah forces in Syria.