Saudi Arabia has agreed to "spend the necessary money" to rebuild Syria, US President Donald Trump said, only a few days after a contentious decision to withdraw US forces from the war-torn country.

Without revealing the details of the supposed agreement or citing official Saudi statements, Trump expressed his gratitude to the Gulf kingdom on Twitter on Monday, writing: "Thanks Saudi A!"

Saudi Arabia has now agreed to spend the necessary money needed to help rebuild Syria, instead of the United States. See? Isn’t it nice when immensely wealthy countries help rebuild their neighbors rather than a Great Country, the U.S., that is 5000 miles away. Thanks to Saudi A! — Donald J. Trump (@realDonaldTrump) December 24, 2018

The White House is facing growing domestic and international criticism over a decision to pull US troops out of Syria, with Secretary of Defence James Mattis announcing his resignation last week in protest of the decision.

In contrast with Trump's claim that the Islamic State (IS) group has been defeated in Syria, Pentagon officials had stressed earlier this year that the fight against IS isn't over.

It is not clear what Trump was referring to on Monday when he thanked Riyadh for its alleged pledge to fund rebuilding the country.

Saudi Arabia has not announced any recent aid initiatives for Syria, other than limited food assistance for 6,000 people outside Aleppo.

In August, Riyadh said it would contribute $100m to northeast Syria for “stabilisation projects” in areas once held by IS and now controlled by US-backed forces, but it remains unclear if Trump was referring to this assistance in his tweet.

You mean the “Saudi A” that murdered @washingtonpost writer Jamal #Khashoggi, kidnapped Lebanon’s prime minister, and pounded Yemen into a humanitarian mess?



It would be nice if the leaders of the United States would stand up for basic humanity instead of giving cover to MBS. https://t.co/TZO3KBNzbX — Karen Attiah (@KarenAttiah) December 24, 2018

With the conflict ongoing, there also has not been any new major push for assistance for reconstruction efforts in Syria.

Still, Trump has long floated the idea of Washington's regional allies filling the void left in the Middle East by the US as it pulls assistance and scales back its military presence in the region in pursuit of his "America First" policies.

"We have asked our partners to take greater responsibility for securing their home region, including contributing large amounts of money for the resources, equipment, and all of the anti-[IS] effort," Trump said in April after ordering military strikes against Syrian government targets.

"Increased engagement from our friends, including Saudi Arabia, the United Arab Emirates, Qatar, Egypt, and others can ensure that Iran does not profit from the eradication of [IS]."