President Trump has definitively agreed to keep some U.S. military forces in Syria, in an apparent partial reversal of his commitment two months ago to fully withdrawing from the country, a National Security Council source tells Fox News.

Amid a bipartisan backlash and concern for local Kurdish forces aligned with the U.S., the White House later said that it would keep a force of approximately 200 troops in Syria for the forseeable future as part of an international contingent -- a decision that a bipartisan group of senators and House members praised in a Feb. 22 letter to Trump. Fox News obtained a copy of that letter on Tuesday.

"Like you, we seek to ensure that all of the gains made in Syria are not lost, that ISIS never returns, that Iran is not emboldened, and that we consolidate our gains and ensure the best outcome in Geneva for American interests," the group, including South Carolina Republican Sen. Lindsey Graham, wrote to Trump.

"I agree 100%," Trump wrote on the letter, near his signature. "ALL is being done."

In December, Trump invoked fallen troops in justifying a pullout from Syria -- and declared victory over ISIS.

"We've been fighting for a long time in Syria," Trump said in a video posted on social media. "I've been president for almost two years, and we've really stepped it up. And we've won against ISIS. We've been them, and we've beaten them badly. And now it's time for our troops to come back home."

He continued: "I get very saddened when I have to write letters or call parents or wives or husbands of soldiers who have been killed fighting for our country. It's a great honor; we cherish them. But it's heartbreaking. Now we've won -- it's time to come back. They're getting ready. You're going to see them soon."

Tuesday's developments come as Islamic State (ISIS) militants fight to hold onto a last tiny piece of territory on the riverside in eastern Syria, deploying snipers, guided missiles and surprise tunnel attacks. The resistance prompted a pounding on Sunday by the U.S-led coalition and its ground allies in their final push to end the extremist group's territorial hold.

Rings of black smoke billowed over the besieged speck of land in the village of Baghouz after airstrikes hit several targets.

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Mortar rounds from a hill overlooking a tent encampment where the militants are still holed up rang into the night.

The U.S.-backed contingent -- Syrian Democratic Forces (SDF) -- resumed an offensive to recapture the area in Baghouz on Friday night, after a two-week pause to allow for the evacuation of civilians from the area. Retaking the sliver of land would be a milestone in the devastating four-year campaign to end IS' self-proclaimed Islamic caliphate that once straddled vast territory across Syria and Iraq.

Some experts, however, are warning of a return of ISIS forces following any U.S. departure, or any power or territorial vacuum that might be created in the future.

The U.S.-backed SDF – which Turkey strongly opposes because of its link to the separatist Kurdistan Workers Party (PKK) -- is so far not part of ongoing U.N.-led talks to end the Syrian conflict, United Nations officials have said. There is concern that the Turks will move against the SDF once the U.S. departs, though Trump administration officials have called for the Kurds to be protected.

Meanwhile, the Red Cross' U.N. observer says the weak and fragile services at the al-Hol camp in northeastern Syria are on "the brink of collapse" because of a flood of civilians and some Islamic State extremists.

Robert Mardini of the International Committee for the Red Cross said in New York on Monday that the population in the camp has risen from 34,000 at the beginning of December to 45,000 now. He said several thousand mainly women and children arrive daily and there is "no end in sight."

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Mardini says those coming out of the ISIS pocket are "sick, injured, tired, afraid," while the situation at the camp is "overwhelming."

He also said the international Red Cross is ready to help with any repatriations of ISIS fighters and their families, provided agreements are negotiated with their home countries. Mardini called this "one of the most complex issues, not only from a humanitarian sense but also from whether countries want their nationals to return or not."

Fox News' Elizabeth Llorente and The Associated Press contributed to this report.