Russia's alliance with Turkey is being tested by the ongoing presence of militant groups in Idlib and attacks on Syrian Kurds in the north.
Russia Foreign Minister Sergey Lavrov said the U.S. is "illegally trying to create a quasi-state" for Kurds in eastern Syria.
The deals may be in jeopardy after local challenges emerged to two precarious international arrangements designed to avoid new violence.
"There is an understanding that this country, which is a U.N. member, has the right to choose its path," said Russia's deputy foreign minister.
An Iranian missile strike targeting jihadis just a few miles from U.S. troops in Syria was a response to warnings from the Trump administration.
In addition to the S-300 missile systems, Russia Defense Minister Sergei Shoigu said Moscow has improved its electronic warfare abilities.
Bahrain's foreign minister surprised observers by saying that "the Syrian government is the ruler in Syria, and we work with countries even if we disagree with them."
"I know that the regime cannot be trusted," Jude Ash, a Syrian activist who is now an asylum applicant in the U.S., told Newsweek. "Any attempt by the regime to facilitate any transition or resolution, the main problem there is trust," he explained.
The deputy director of Russia's state-run Concern Radio-Electronic Technologies said Moscow would soon be able to track planes, "be it in Israel or Saudi Arabia or even in Europe."
Iran's top security official said Israel "will face regrettable reactions" if it tries to attack Syria again, as Russia's strategic balance sways.
While the president criticized Russia and Iran during the Security Council meeting, he expressed appreciation for the "restraint" both nations have shown in Syria's Idlib province.
Russia's decision to equip its Syrian ally comes after an Israeli air raid and amid U.S. threats of military action.
A deal struck Monday by Russia and Turkey appears to have stalled any sort of imminent Syrian military attack on Idlib, but tensions remain high in the region.
Vladimir Putin and Recep Tayyip Erdogan agreed to establish a demilitarized zone of 15 to 20 kilometers in Idlib.
The U.S. conducted a major live-fire exercise with Syrian rebel allies, while warning Russia and Iran not to support the government against insurgents in Idlib.
Western powers may not even wait for a chemical weapons attack to strike Syria, whose allies are rallying to resist.
The U.S. has already entered talks with France and the U.K. in what Russia fears could be a new set of strikes against its Syrian ally.
The U.N. has warned an Idlib offensive could cause the worst humanitarian catastrophe of the 21st century.
Two reports, one of which may have been censored by the Israeli military, claim Israel covertly armed and funded Syrian rebels for years.
After years under siege by their own government, wounded Syrians found unlikely aid from a longtime foe: Israel.
Bob Woodward's "Fear: Trump in the White House" will be released on September 11.
Up to 2 million civilians and opposition fighters await a Syrian military assault on the Islamist-held province of Idlib, a move championed by Russia and criticized by the West.