U.S. intelligence agencies are closely monitoring suspected chemical weapons storage sites in Syria, looking for indications that forces loyal to President Bashar al-Assad are preparing to employ them against the collection of rebel groups fighting to depose him, officials said Saturday.

The agencies assess that Mr. al-Assad’s forces have maintained limited stockpiles of chemical weapons, including munitions loaded with the nerve agent sarin, and there is growing concern that the government could employ them as part of a last-ditch effort to prevent rebels from seizing the capital, Damascus, said the officials, who spoke on condition of anonymity to discuss intelligence matters.

Mr. al-Assad’s government has repeatedly used chemical weapons, including nerve agents and chlorine gas, against rebels and his own people during the 13-year civil war, according to assessments by human rights monitors, the United States and others.

Key Arab allies of the United States want to keep Mr. al-Assad in power because they fear that if the collection of rebel groups topples the government in Damascus, the country could become a more dangerous haven for terrorism. While many of those allies have opposed Mr. al-Assad in the past, they see him as a known quantity and better than the rebel-led alternative, a senior Biden administration official said.

Aides to President Biden have made clear in recent days that the United States has no intention of intervening to affect the war’s outcome, either in support of the rebels or Mr. al-Assad.

That message was echoed on Saturday by President-elect Donald J. Trump, who wrote in a post on his social media platform, Truth Social, “Syria is a mess, but is not our friend, & THE UNITED STATES SHOULD HAVE NOTHING TO DO WITH IT. THIS IS NOT OUR FIGHT. LET IT PLAY OUT. DO NOT GET INVOLVED!”