There was no immediate confirmation from the Damascus regime.
A source with pro-regime forces told AFP: "The Syrian army has taken protective measures especially for military airports and bases." The source said they expected the Americans would inform the Russians before any strike.
Last year, Trump launched a cruise missile strike against a Syrian air base in retaliation for a sarin attack the United Nations later pinned on Assad.
More than 350,000 people have been killed in Syria's conflict since it started in 2011 with the brutal repression of anti-government protests.
It has since spiralled into a complex war involving world powers.
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