State Department spokeswoman Marie Harf could not tell reporters on Monday how many American civilians are thought to be caught up in the conflict, as citizens of other nations take to ships.

But she said US authorities would contact those that it could, and advise them of opportunities to board ships from allied nations docking to pick up their own nationals.

"At this point, we have encouraged all US citizens to shelter and secure locations until they're able to depart safely. The airports are still closed, is my understanding," she said.

A protest website representing Yemeni Americans – -- has accused Washington of abandoning dual nationals last month it its haste to close its embassy in Yemen.

Asked about a claim that 200 US citizens have signed up to the site, Harf said officials had been unable to confirm the figure and urged them to sign on to a State Department list.

France has evacuated 63 foreign nationals from the Yemeni port of Balhaf and frigates from the Chinese and Indian navies have rescued 450 and 100 people respectively from Hodeida.

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