A “massive influx” of thousands of Yemeni refugees is expected in the Horn of Africa as fighting escalates between rebels and troops loyal to the ousted president.

The UN estimates that clashes between the rebels, who are Shias from the Houthi tribe, and the Sunni supporters of ex-president Abd Rabbuh Mansur Hadi have killed more than 500 people and left 1,700 wounded in less than two weeks.

As a result, the first Yemeni refugees have already made the arduous crossings to Djibouti and Somalia’s autonomous region of Somaliland, the United Nations High Commissioner for Refugees (UNHCR) has confirmed.

“We were planning for 5,000 before the escalation of the conflict so now we have to review this number, it may be a lot higher,” revealed Frédéric Van Hamme, external relations officer at UNHCR’s Djibouti base, who said the UN and the government in Djibouti are stepping up preparations to accommodate the larger than expected migration.

“We are expecting a massive influx. The government is using these words,” he added. “We had a meeting with the prime minister [of Djibouti], the minister of interior and the minister of health, they were saying to prepare for an influx, but the exact extent is difficult.”

The refugees, mainly from the south of Yemen, began to arrive after the launching of Saudi-led Operation Decisive Storm on 25 March. The Sunni Saudis have been supporting the forces loyal to President Hadi, who has fled the country, against the rebels, who they claim are backed by Shia Iran.

Djibouti has a population of just 870,000 people and a large influx would put a huge strain on its resources, adds Van Hemme. “For a small country like Djibouti, it’s very difficult, the pressure this puts on the country. If you are looking at a massive influx, the needs are huge.”

However, refugee sources speaking to UNHCR said they had no other option but to flee to Djibouti. Describing the fighting, Adel Ali Atya, a fisherman and father of four, said: "The plane started to launch missiles from 8pm until sunrise. At 9am, we left Bab el Mandouk [the island] and we arrived in Obock [In the north of Djibouti] by 7pm. If the war will calm down, we can go back. If the war doesn't calm down, we have to stay here. We have nowhere else to go."

Many people from villages in southern Yemen have moved internally to mountainous areas within the country, while those who arrived in Djibouti said the ticket to the Yemeni highlands was more expensive (at approximately 40,000 Yemeni rials or £124) than fleeing by boat.

In contrast to Djibouti, which has identified a site in Obock for construction of a refugee camp, Somaliland will construct camps upon arrival of the expected Yemeni refugees. Ali Saeed Raygal, the autonomous region’s resettlement minister, said: “When they come, then we build an area of camps. The first area is Berbera Port but we are still not sure how many people are coming from Yemen. After two, three days we should know how many.”

He added that he believed “maybe 2,000, up to 3,000” would make the trip across the Gulf of Aden.

The direction of flight, however, has traditionally been in the other direction; UNHCR figures show that 95% of Yemen’s 246,000 registered refugees are Somali nationals.