Sir Michael said action should be justified, legal and proportionate but stated that in those circumstances the UK's support for US action by Donald Trump would be unwavering.
It comes after the White House has said it has "potential" evidence that Mr Assad's military forces are preparing for another chemical attack on rebel forces.
Sir Michael told BBC Radio 4's Today programme: "As always in war, the military action you use must be justified, it must be legal, it must proportionate, it must be necessary. In the last case it was. If the Americans take similar action again, I want to be very clear, we will support it."
He said the US has been monitoring the situation in Syria, but admitted it had not shared any specific evidence with the British Government about the regime preparations.Sir Michael explained that he expected to discuss the current position with US Defence Secretary General James Mattis when they meet at a gathering of Nato defence ministers later this week.
He added: "Jim Mattis did review the various options with me last time before the final meeting with the President. I shall be seeing Secretary Mattis at the Nato meeting on Thursday. This is one of the matters we expect to review."
In a statement released late on Monday, the White House said preparations being taken by Syria were similar to those undertaken before an 4 April chemical attack that killed dozens of civilians and prompted Mr Trump to order a cruise missile strike on an air base.
White House spokesman Sean Spicer added: "The United States has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children. If ... Mr Assad conducts another mass murder attack using chemical weapons, he and his military will pay a heavy price."
Sir Michael pointed out that Britain had backed the US administration when it mounted missile strikes against the regime following a chemical attack last April. The strike on the Shayrat airfield in April came after a poison gas attack that killed 87 people in rebel-held territory, with the West blaming the Assad regime.
The Syrian ruler claimed in an interview that the April attack was a "100 per cent fabrication" used to justify a US air strike.
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