More than 300 Yemenis fleeing violence at home have arrived in Djibouti, an official with the U.N. refugee agency said on Thursday, and other refugees were seeking shelter elsewhere in the Horn of Africa.

The agency has already registered 243 people, most of them fishermen, and 65 more people arrived on Thursday, said Frederic Van Hamme.

The refugees, who are crossing the Gulf of Aden by boat, say the journey is difficult, with a lack of fuel and boats as well as efforts by some at home to prevent people from fleeing, he said.

Van Hamme said the agency initially had a contingency plan in place for 5,000 people but is now revising the number upward.

Other Yemenis have sought refuge in Eritrea and Somalia, reversing a trend in which refugees fleeing violence and poverty in Somalia and Eritrea have crossed the waters into Yemen.

The International Committee of the Red Cross is scaling up its operations in Djibouti in anticipation of a possible influx of Yemeni refugees, said Jason Straziuso, the agency's spokesman in East Africa.

An ICRC boat carrying a surgical team of four people arrived in the Yemeni city of Aden on Wednesday to offer advanced surgical care for wounded and sick people, he said.

At least 643 civilians and combatants have been killed in fighting in Yemen since March 19, according to the World Health Organization.

Forces loyal to President Abed Rabbo Mansour Hadi, who fled the country two weeks ago, are fighting rebels and allied military units who back Ali Abdullah Saleh, an ousted former president. Rebel positions are also being targeted by Saudi-led air strikes.