In support of the Idlib operation, Russia has deployed a significant naval presence in the eastern Mediterranean, with weaponry capable of reaching across Syria.

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The Marine exercise took place near the U.S. garrison at Tanf, along the Syrian-Iraqi border near Jordan, around which the Americans have long declared a 35-mile deconfliction zone off-limits to others. It followed a Russian notification, and U.S. rejection, of a plan to enter the zone to pursue “terrorists.”

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“The United States does not seek to fight the Russians, the government of Syria or any groups that may be providing support to Syria in the Syrian civil war,” Lt. Col. Earl Brown, a U.S. Central Command spokesman, said in a statement. “However, the United States will not hesitate to use necessary and proportionate force to defend U.S., coalition or partner forces.”

The garrison has served as a launchpad for U.S. anti-Islamic State operations and a U.S. Special Operations Forces training facility for Syrian fighters aiding in that effort. The zone also lies astride a major transport line between Tehran and Damascus, and control of the area has long been a goal of the Syrian government and its Russian and Iranian allies.

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Increasingly frequent skirmishes have occurred near and inside the zone, including one Monday in which several Iranian and Iranian-backed fighters were reportedly killed in an airstrike. A U.S. military spokesman said the base had “received fire from unknown forces” but “did not fire back.” Israel, which has conducted strikes against Iranian forces in Syria, made no direct comment on the Monday incident.

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Russia has repeatedly charged that the United States is harboring Islamic State forces inside the zone, something the Americans have vehemently denied while suggesting that Russia and its allies in Syria are looking for an excuse to attack the area.

“Coalition partners are in the At Tanf deconfliction zone for the fight to destroy ISIS,” Brown said. “Any claim that the U.S. is harboring or assisting ISIS is grossly inaccurate.” ISIS is an acronym for the Islamic State.

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Outlining an unusual series of events that led up to the exercise, Brown said that Russia, using a communication line dedicated to avoiding accidental conflict between U.S. and Russian forces in Syria, had notified U.S. Lt. Gen. Paul E. Funk II, the commander of coalition forces in Iraq and Syria, that “they intended to enter the At Tanf deconfliction zone to pursue terrorists.”

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That was followed Thursday, Brown said, by an unprecedented written note to Funk from his Russian counterpart “that they would make precision strikes.”

“The U.S. does not require any assistance in our efforts to destroy ISIS in the At Tanf deconfliction zone,” Brown said, “and we advised the Russians to remain clear.”