Syria and Russia condemned the United States’ military presence in Syria as “illegal” Thursday after an overnight confrontation in which U.S. warplanes bombed pro-Syrian-government forces as they approached an American-supported base.

U.S. forces targeted the pro-government troops with airstrikes and artillery after they launched an attack against a base belonging to the U.S.-backed Syrian Democratic Forces in the eastern province of Deir al-Zour, according to Col. Thomas F. Veale, a U.S. military spokesman. He said U.S. military personnel advising the Kurdish-led SDF were at the base at the time.

The Syrian government accused the United States of “aggression” in launching the strikes, which it said killed “scores” of people. Russia denounced the U.S. presence in Syria as “illegal” and accused the United States of seeking to seize Syria’s oil.

“The recent incident once again shows that the United States’ illegal military presence in Syria is actually aimed at taking control of the country’s economic assets and not at fighting against the ISIS international terror group,” the Russian Defense Ministry said in a statement, referring to the Islamic State.

There were no U.S. casualties, and one SDF fighter was injured in the three-hour battle, during which the United States called on Apache helicopters, AC-130 gunships and F-15 fighter jets to repel the assailants, U.S. military officials said.

On Jan. 17, Secretary of State Rex Tillerson outlined a range of U.S. policy goals in the Syrian conflict, including an "enduring defeat" of "ISIS and al-Qaeda." (Department of State)

In Washington, Defense Secretary Jim Mattis described the situation as “perplexing,” and said he had “no idea why they would attack” the base. Both Russian and Syrian-aligned forces on the ground had long known the U.S. and allied forces were there, he said. Russia was immediately contacted via existing lines of communication, but “we’ve always known that there are elements in this complex battle space that Russia did not have control of,” Mattis said.

It was the most serious clash involving the United States and Syria since U.S. troops began deploying to northeastern Syria in late 2015 in support of Kurdish and Arab fighters against the Islamic State. It illustrated the complexity of the battlefield now that the war against the extremist group is winding down, leaving the United States in overall control of about a quarter of Syria.

The Syrian government and its ally Iran have repeatedly called for U.S. troops to leave Syria now that the fight against the Islamic State is nearly over, and they have regularly threatened to wage war to push the Americans out if they do not leave.

However, the Trump administration announced a new Syria strategy last month under which U.S. troops will remain in northeastern Syria until there is a peace settlement regarding the wider war that includes a transition away from the rule of President Bashar al-Assad.

With no sign of such a peace deal in sight, the U.S. military is now committed to a potentially indefinite presence in Syria that is opposed by all of the main players in the country.

The prospect that U.S. troops will remain in Syria while shoring up Kurdish efforts to secure self-rule has provoked a convergence among countries opposed to any form of Kurdish autonomy, uniting Turkey, Russia, Iran and the Syrian government in a de facto alliance against the U.S. presence.

The Syrian government said it captured the eastern city of Deir al-Zour, while the Iraqi military said its forces entered the border town of Qaim. (The Washington Post)

The United States “is now in mission-creep mode,” said Robert S. Ford, a former U.S. ambassador to Syria who is now a senior fellow at the Middle East Institute.

“The Americans have managed through their diplomatic strategy to isolate themselves to the point where Turkey, Iran and Syria all agree that what the U.S. is doing in Syria is bad,” he said.

Many in Congress also are starting to question whether the extended U.S. presence in northeastern Syria risks embroiling the United States more deeply in Syria’s war. The confrontation “raises serious questions about our continued presence in Syria,” Sen. Tim Kaine (D-Va.), a member of the Armed Services Committee, said in a statement. “I am gravely concerned that the Trump Administration is purposefully stumbling into a broader conflict, without a vote of Congress or clear objectives.”

Mattis denied that the attack and the U.S. response constituted American engagement in the Syrian civil war. “We are there to fight ISIS,” he said. “That’s what those [U.S.] troops were doing in that position.” If the Americans were “getting involved in the broader conflict, we would have moved to the other side” of the river and continued to engage the pro-government forces. He said the remaining attackers retreated to the western side of the Euphrates.

But the war continues to become ever more complicated. The attack on the southernmost flank of the SDF-controlled area in northeastern Syria came as Turkey pressed ahead with its nearly three-week-old offensive against the Kurdish enclave of Afrin in northwestern Syria. The enclave is controlled by the same Kurdish force that dominates the SDF, but it does not have direct U.S. support.

Turkey also has been threatening to attack the U.S.-backed forces farther east, around the town of Manbij, where U.S. troops mount regular patrols. The attack in Deir al-Zour coincided with a visit by Lt. Gen. Paul E. Funk II, commander of U.S. forces in Iraq and Syria, to the front lines in Manbij, where he offered assurances to the SDF of U.S. support.

The Kurdish-led force is feeling increasingly “under attack from all sides,” said Kino Gabriel, a spokesman for the SDF in northeastern Syria. “This is an attack on the political project we are trying to establish in Syria.”

If the tensions persist, the SDF and its U.S. military advisers could soon find themselves fighting a war on three fronts — against Turkey, the Islamic State and the Syrian government alliance. But, Gabriel said, the SDF is confident that the force has enough fighters to confront the Turkish incursion and hold its lines in Deir al-Zour.

He said the attack on the SDF base was conducted by members of the National Defense Forces, a Syrian militia that is loyal to the Syrian government and which has received arms and supplies from Iran and the Lebanese Hezbollah movement. The likely goal, he said, was to attempt to halt any more SDF advances farther east, where only a few Islamic State-held villages lie between SDF positions and the Iraqi border.

“We assume it is because they want to divert our attention from the ongoing operations against Daesh in the Euphrates Valley,” he said, using an Arabic acronym for the Islamic State. “We only have a few more towns and villages to be cleared of ISIS. They are completely surrounded, and we are just waiting to finish our operations and clear the ­Iraqi-Syrian border.”

Since last year, the Euphrates River has served as a deconfliction line, separating the United States and its SDF allies on one side, and Russia and its Syrian government allies on the other. Negotiated between the U.S. and Russian militaries last year, until now it has served mostly to keep the rival armies apart.

Veale, the U.S. military spokesman, said the hostilities erupted after several hundred infantry troops backed by tanks and artillery began advancing on an SDF position in the town of Khasham, about five miles east of the deconfliction line. It lies near some of Syria’s biggest oil fields under SDF control, and he speculated that the pro-government forces were trying to take them back.

When the advancing forces unleashed a volley of 20 to 30 tank and artillery rounds toward the base, U.S. warplanes and artillery struck back, Veale said in an emailed statement. “This action was taken in self-defense,” he said.

Channels of communication between the U.S. military and Russia remained open throughout the clash Thursday night, U.S. officials said, and the Americans informed Russia before they opened fire on the pro-government force.

The Russian Defense Ministry, however, said the battle occurred when a reconnaissance party made up of Syrian militia fighters crossed the Euphrates River to hunt down an Islamic State position, only to be attacked unprovoked by U.S. warplanes. The militias had neglected to inform the Russian military of their plan in advance, the Russian statement added

Missy Ryan and Karen DeYoung in Washington contributed to this report.