National security adviser John Bolton warned Monday that a Russian delivery of a missile defense system to the Syrian government is a substantial escalation.
"We think introducing the S-300s [missile defense systems] 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 said, according to The Associated Press.
Russia announced earlier Monday that it would give the Syrian government the S-300 after Syria's current missile defense systems accidentally downed a Russian plane last week.
Russia blamed the incident on "misleading information" from the Israeli air force.
Russian military spokesman Igor Konashenkov said that Israeli command warned on the day of the incident that it would strike the "north of Syria," according to Agence France-Presse.
Konashenkov said Russia ordered its plane to return to its base, but "one minute" after the call, Israeli F-16s struck targets in western Syria, according to the news service.
Israel has issued a full-throated denial of the accusation.
"We have American forces that we're concerned about," Bolton said Monday. "The Israelis have a legitimate right to self-defense 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.
Tensions continued to worsen between the various players in Syria as the U.S. and allies prepare to respond to a possible chemical weapons attack on the remaining rebel forces in Idlib.
Secretary of State Mike PompeoMike PompeoSunday shows preview: Coronavirus dominates as country struggles with delta variant Christie, Pompeo named co-chairs of GOP redistricting group America needs a new strategy for Pacific Island Countries MORE warned Sunday that the U.S. will hold anyone who uses chemical weapons in Syria accountable.
When asked if that might include the use of military force, Pompeo responded that they aren't ruling out "a single thing."
Earlier this month, the chairman of the Joint Chiefs of Staff, Marine Gen. Joseph Dunford, indicated that President TrumpDonald TrumpKinzinger says Trump 'winning' because so many Republicans 'have remained silent' Our remote warfare counterterrorism strategy is more risk than reward Far-right rally draws small crowd, large police presence at Capitol MORE had been presented with military options for Syria.
The Wall Street Journal also reported several weeks ago that Trump privately threatened to retaliate with a massive attack if Syrian President Bashar Assad used chemical weapons.
When asked two days later if America would respond with military force, Defense Secretary James MattisJames Norman Mattis20 years after 9/11, we've logged successes but the fight continues Defense & National Security — The mental scars of Afghanistan House panel advances 8B defense bill MORE said, "I am not going to give that clarity."
The Washington Post reported three weeks ago that the president has approved stationing about 2,000 U.S. troops in Syria indefinitely.
According to The Associated Press, Bolton said Monday that the U.S. military will not leave Syria until Iran is no longer active in the country.
"We're not going to leave as long as Iranian troops are outside Iranian borders and that includes Iranian proxies and militias," Bolton said.