The United States says the Russia decision to supply Syria with an advanced air defence missile system is a "major mistake".
Moscow said on Monday it will bolster Syria's air defence following the downing of a Russian plane last week by sending a modern S-300 system to the Syrian army within two weeks.
"We think introducing the S-300s to the Syrian government would be a significant escalation by the Russians and something that we hope, if these press reports are accurate, they would reconsider," US National Security Advisor, John Bolton, said on Monday.
"We have American forces in the area we're concerned about," Bolton said.
"The Israelis have a legitimate right to self-defence against this Iranian aggressive behavior, and what we're all trying to do is reduce tensions, reduce the possibility of major new hostilities. That's why the president has spoken to this issue and why we would regard introducing the S-300 as a major mistake." he added.
In a televised statement on Monday, Russia's Defence Minister Sergei Shoigu said the decision to "transfer the missile defence system to the Syrian armed forces "was taken by President Vladimir Putin and is one of the "response measures".
On the same day, the Kremlin accused Israeli pilots of "premeditated actions" over the downing of a Russian plane in Syria last week, warning it will harm 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," Kremlin spokesman Dmitry Peskov told journalists.