Washington (CNN) Military commanders warned President Donald Trump on December 26 during his surprise visit to Iraq that -- despite his claims to the contrary -- ISIS was not entirely defeated in Syria.

People familiar with the President's reaction said the conversation was eye-opening for a leader who days earlier claimed the terror group was defeated "badly" in the country.

The warning comes into new light with the news that a suicide blast in northern Syria killed four Americans as Trump presses forward with a withdrawal of US forces from the country.

The terror group has claimed responsibility for the massacre in Manbij, a northern city that American-backed forces had worked to clear of Islamic State fighters. That did not stop Vice President Mike Pence, who was briefed on the attack earlier in the day, from declaring ISIS "defeated" during morning remarks.

The December discussion occurred inside a tan tent at the al-Asad airbase west of Baghdad and included the US Ambassador to Iraq Douglas Silliman, Lieutenant General Paul LaCamera, national security adviser John Bolton, and the first lady Melania Trump, along with other officials.

Trump was informed that pockets of ISIS militants remained in the Euphrates River valley and that US military had not yet eliminated all of their strongholds. Commanders told him the US had been successful in taking back other areas but that the job was not finished.

The latest bombing presents a stark reminder for Trump and his administration of the continued violence in Syria, even as he insists US troops begin returning home. Trump initially announced in December that American servicemen and women would begin withdrawing immediately, against the advice of many of his national security advisers.

Since then, however, the administration has seemed to waver on the immediacy of the withdrawal. National security adviser John Bolton last week listed conditions for pulling out US troops that could take many months.

And Trump has taken a more cautious approach after being cautioned by top military brass that a complete and sudden withdrawal could lead to an ISIS resurgence.

Sobering talk

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The people familiar with the December conversation described it as sobering, and said it broke through to Trump in a way his conversations with national security officials in Washington had not. Coming days after Trump's decision to withdraw troops from Syria, it centered on the remaining challenges of going after ISIS fighters in pockets of Syria.

In addition to the briefing from the commanders, Trump found the massive security apparatus that surrounded the trip surprising -- something his advisers told him was reflective of the danger still posed by ISIS.

While the trip proved illuminating for the President, however, it did not shake him of his conviction that US troops should begin returning after years of overseas engagements, and there were few indications on Wednesday that the attack in Manbij would throw him from his goal.

"The President has been fully briefed and we will continue to monitor the ongoing situation in Syria," press secretary Sarah Sanders told reporters early in the day.

Speaking at the State Department hours later, Pence reiterated the President's goal of returning troops home.

"Thanks to the leadership of this commander in chief and the courage and sacrifice of our armed forces we're now actually able to hand off the fight against ISIS in Syria to our coalition partners and we are bringing our troops home," he said. "The caliphate has crumbled and ISIS has been defeated."

Over the weekend, the President also reinforced his view that American troops should return home, though acknowledged ISIS remains in some parts of the country.

"Starting the long overdue pullout from Syria while hitting the little remaining ISIS territorial caliphate hard, and from many directions," Trump wrote on Twitter this weekend. "Will attack again from existing nearby base if it reforms. Will devastate Turkey economically if they hit Kurds. Create 20 mile safe zone."

"Likewise, do not want the Kurds to provoke Turkey," he went on. "Russia, Iran and Syria have been the biggest beneficiaries of the long term U.S. policy of destroying ISIS in Syria - natural enemies. We also benefit but it is now time to bring our troops back home. Stop the ENDLESS WARS!"

Questions remain

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Still, neither Trump nor the administration has said who will enforce or finance the so-called safe zone. And many in the administration have privately said a US presence in Syria is needed to counter Iran and Russia.

Sen. Lindsey Graham, a presidential ally who was critical of Trump's original announcement to withdraw US troops from Syria, said Wednesday he is concerned Trump's statements on Syria may have encouraged the enemy.

"My concern, by the statements made by President Trump, is that you set in motion enthusiasm by the enemy we're fighting," he said during a Judiciary Committee hearing on Attorney General nominee William Barr. "You make people we're trying to held wonder about us. And as they get bolder, people we're trying to help are going to get more uncertain. I saw this in Iraq. And I'm now seeing it in Syria."

"I would hope the President would look long at hard at where he's headed in Syria," Graham said. "I know people are frustrated, but were never going to be safe here unless we're willing to help people over there who will stand up against this radical ideology."