U.S. Defense Secretary James Mattis said Russia's expected delivery of an advanced anti-aircraft missile system to Syria would be a "tragedy" for peace-making efforts in the war-torn country.
"I think that anything that is done to continue this fighting in Syria is actually a tragedy," Mattis told a Senate Armed Services Committee hearing Thursday, ahead of talks with Israeli Defense Minister Avigdor Lieberman in Washington.
"We need to all get behind the Geneva Convention process and come to a diplomatic solution for this," he said.
Russian Colonel General Sergei Rudskoi told reporters on Wednesday that Moscow would send a team of military trainers to Syria ahead of the expected delivery of the S-300 weapons system.
Israel believes the anti-aircraft system would hamper its ability to strike targets inside Syria and would bolster Iran's influence there.
Mattis told the hearing he expects a "re-energized" effort against the Islamic State group in eastern Syria in the coming days.
"You'll see a re-energized effort against the middle Euphrates River Valley in the days ahead and against the rest of the geographic caliphate," Mattis told the committee.
U.S. airstrikes, troops and U.S.-backed Syrian militias have dealt heavy blows to Islamic State in Syria, but the group still holds some areas and is widely expected to revert to guerrilla tactics if the last remnants of its once self-declared "caliphate" are captured.
U.S. officials have said that in recent days they have seen fighters from the U.S.-backed Syrian Democratic Forces, an alliance of militias in northern and eastern Syria dominated by the Kurdish YPG, returning to the middle Euphrates River Valley to fight against Islamic State.
A Turkish offensive in Afrin against the Kurdish YPG militia, which Ankara regards as an extension of a Kurdish insurgency at home, led to an "operational pause" in the fight against Islamic State in eastern Syria in March.
Mattis added that he was also expecting increased operations against Islamic State on the Iraqi side of the border. He said that French special forces have reinforced the fight against the Islamist group in Syria over the past two weeks as well.
U.S. President Donald Trump on Tuesday said he wants to withdraw American troops from Syria "relatively soon" but not before their mission is complete, adding that negotiations over the crisis there should be part of a larger deal regarding Iran.
Mattis said on Thursday the United States would "probably regret" not keeping a holding force in Syria to ensure that Islamic State did not re-emerge and that any withdrawal of U.S. forces was based on conditions.
"We have to create local forces that can keep the pressure on any attempt by ISIS to try to [re-emerge]," Mattis said.
Asked whether it would be risky to have local holding partners without U.S. forces, Mattis said, "I am confident that we would probably regret it."
