NEW YORK (Reuters) - U.S. National Security Adviser John Bolton said on Monday that Russian plans to supply Syria with an S-300 missile system would be a “significant escalation” by Moscow and that he hopes it will reconsider.

National Security Adviser John Bolton discusses "Protecting American Constitutionalism and Sovereignty from International Threats," at a forum hosted by the Federalist Society for Law and Public Policy Studies in Washington, U.S. September 10, 2018. REUTERS/Eric Thayer

Russia said on Monday it will supply the surface-to-air missile system to Syria in two weeks against strong Israeli objections, a week after Moscow blamed Israel for indirectly causing the downing of a Russian military plane in Syria.

“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,” Bolton told reporters.

“There shouldn’t be any misunderstanding here ... The party responsible for the attacks in Syria and Lebanon and really the party responsible for the shooting down of the Russian plane is Iran,” he said.

Russia is fighting in Syria in support of the government.

Bolton warned that if the Syrian government used chemical weapons again, the U.S. response would be significant.

“If in fact they make the mistake of using chemical weapons again, the retaliation would be much stronger than before and would have the intended effect of creating structures of deterrence that they never do it again,” he said.

Bolton said the United States was doing everything it could to ensure Syria did not use chemical weapons and had pressed Russia about the issue as well.

Bolton said a political process was needed but said Russia’s plans on the missile system made that difficult. He said U.S. troops would stay active in Syria as long as Iran was involved.

“We’re not going to leave as long as Iranian troops are outside Iranian borders, and that includes Iranian proxies and militias,” he said.