WASHINGTON (AFP) - Hundreds of American citizens and their families have fled Yemen on foreign ships, joining an exodus of terrified people fleeing intense airstrikes against Shiite rebels, a US official said Tuesday.

They have been brought to safety on board Indian, Korean and Russian ships sailing from Yemen across a narrow channel of the Red Sea, only 30 kilometers (20 miles) wide, to Djibouti in the Horn of Africa.

Last week the United States admitted it had no immediate plans to rescue its nationals stranded in war-torn Yemen and urged them to flee the raging conflict by sea - triggering angry claims that Yemeni-US dual nationals had been abandoned by Washington.

Acting State Department spokeswoman, Marie Harf, who called such claims "offensive," said those arriving in Djibouti had been offered food, water and medical attention as well as shelter from the heat by US embassy officials as they waited to be screened and processed.

US embassy staff had also been increased to help process applications by family members of US citizens as quickly as possible.

Homeland Security "has granted exceptional authority for the consular team in Djibouti to accept and approve immigrant visa petitions for spouses, children and parents of US citizens," Harf told reporters.

US ambassador to Djibouti Tom Kelly said in a tweet Tuesday that so far 178 US citizens and 125 non-American family members had arrived in Djibouti on 12 ships and one plane.

"Thank you #Djibouti for your hospitality and humanitarian aid for #Yemenrefugees," he added in another tweet, with a picture of a boat docking.

US officials say there has been concern that any US operation to evacuate its citizens could become a target, amid the chaos in Yemen where Shiite Huthi militias are under attack from a Saudi-led Arab coalition trying to dislodge them.

"We actually think that if a US military asset was brought into the port that would be a significant security risk," a senior State Department official said, adding there was concern such a move "could put everyone at risk."

"We are in contact with other countries, talking to them about what they have available."

Officials from the US embassy in Yemen - which has temporarily relocated to Jeddah - have been posting messages on its website to tell US citizens which boats are leaving and from where.

Some 41 American citizens who had been left stuck in war-torn Yemen announced last week they were suing the United States in a bid to get it to evacuate them.