Regime airstrikes have pounded Syria's Idlib province, with clashes reported between government and rebel ground forces.
The move comes as Kurdish leader Ilham Ahmed's Syrian Democratic Council enters tough negotiations with the Syrian President Bashar al-Assad's government in hopes of finding peace.
Syrian Kurdish fighters left behind by a U.S. exit are looking "for clear mechanisms to protect the northern border" by appealing to Syrian President Bashar al-Assad and Russia.
"ISIS presents a very real threat to the long-term stability in this region and our mission remains the same, the enduring defeat of ISIS," said Major General Christopher Ghika, Operation Inherent Resolve deputy commander.
"We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency," the president tweeted.
General Kenneth McKenzie, President Donald Trump's nominee for CENTCOM chief, said the mission remained limited to defeating ISIS, despite statements from other officials.
The U.S.-led coalition claimed to have killed an infamous ISIS commander, but the Syrian government and activists said no.
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."
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.
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.