Four Americans, including two U.S. service members, were among those killed in an attack in northern Syria Wednesday, the same day Vice President Mike Pence said ISIS, which claimed responsibility for the attack, "has been defeated."

U.S. Central Command said in a statement Wednesday that an apparent explosion killed two service members, a Department of Defense civilian and a Pentagon contractor while they were "conducting a local engagement in Manbij, Syria." Three service members were also injured.

"Initial reports indicate an explosion caused the casualties, and the incident is under investigation," the statement said.

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The attack comes just weeks after President Trump declared ISIS defeated and said U.S. troops were coming home. Speaking to a gathering of U.S. ambassadors at the State Department on Wednesday, after the Pentagon confirmed the deaths of American troops in Syria, Pence repeated both of Mr. Trump's assertions.

This frame grab from video provided by Hawar News, ANHA, the news agency for the semi-autonomous Kurdish areas in Syria, shows a damaged restaurant where an explosion occurred, in Manbij, Syria, Jan. 16, 2019. ANHA/AP

White House press secretary Sarah Sanders told CBS News on Wednesday that Mr. Trump had been "fully briefed and we will continue to monitor the ongoing situation in Syria." She referred all additional questions to the Pentagon.

South Carolina Republican Sen. Lindsey Graham announced he will travel to Turkey Wednesday night on a trip that appears to have been previously scheduled. He told reporters he thinks the attack took place at a restaurant that he and New Hampshire Sen. Jeanne Shaheen, a Democrat, had visited.

"It was a nice restaurant — incredibly welcoming — the people around us there were from different backgrounds trying to hold the city together," he said.

Aides to Shaheen said that based on news footage from the scene, they believe the restaurant is in northern Syria but are waiting for more information from the Pentagon to confirm whether it is. Graham warned, "It breaks my heart if it's the same restaurant. It breaks my heart no matter what happens. But if it's the same restaurant, it tells me all I need to know about what's going on in northeast Syria ... This is the beginning of what happened in Iraq."

Manbij is just 20 miles from the border with Turkey, in an area held by Kurdish forces allied with the U.S. military coalition fighting ISIS. The terror group's self-styled news agency Amaq claimed online that the attack was carried out by a member of the group wearing a suicide vest.

The U.S. has about 2,000 troops still in Syria, though Mr. Trump has ordered a withdrawal that is expected to be completed within four months.

Turkish President Recep Tayyip Erdogan said Tuesday that Mr. Trump had reassured him in a phone conversation the U.S. was in the process of pulling its troops out of Syria, appearing to tamp down tension between the two nations which manifested itself just hours earlier in a testy exchange of tweets.

An image grab taken from a video obtained by AFPTV on January 16, 2019, shows U.S. armoured vehicles at the scene of a suicide attack in the northern Syrian town of Manbij. Getty

Turkey wants the remaining American forces in Syria to come out, which would give the Turks free rein to launch offensive operations against the Kurdish militia in Syria's north. But many of those Kurdish fighters are U.S. allies who have been crucial to the fight against ISIS. The Trump administration has made guarantees for their security a precondition of the complete U.S. withdrawal from Syria.

Erdogan told his nation's lawmakers on Tuesday that he had told Mr. Trump the U.S.-allied YPG Kurdish militia "tortures the groups in Syria that do not depend on them," and that his government had shared its evidence that America's allies are in fact terrorists with the White House.

But the Trump administration has not backed away from its insistence that the Kurds of the YPG be protected. Nor has it officially backed away from Mr. Trump's decision to pull U.S. troops out of Syria — though that process is happening far slower than the president initially suggested.

In the meantime, as CBS News correspondent Charlie D'Agata reported this week, there's been an increase in the intensity of the battle against ISIS holdouts. The YPG-led Syrian Democratic Forces (SDF) are making the most of U.S. military support while they still have it.

Administration officials have stressed the outcome of the battle against ISIS is not reliant on the physical presence of the roughly 2,000 U.S. forces in Syria, but as D'Agata reported, the Americans have played a vital role in the fight on the ground. They provide not only tactics, weapons and equipment, but crucially they also direct airstrikes against ISIS targets.

D'Agata and his team witnessed that for themselves on board the American aircraft carrier USS John C. Stennis last week. From the waters of the Persian Gulf, wave after wave of F-18 fighter jets rocketed into the sky to launch bombing raids against ISIS targets in Iraq and Syria.

The Kurdish fighters who spoke to D'Agata in northern Syria are not only concerned that the withdrawal of U.S. troops could enable an ISIS comeback, but that Turkish forces will go on the attack the moment the last American soldier leaves.

Ed O'Keefe contributed to this report.