ADEN/United Nations - Dozens of Yemeni rebels and pro-government fighters have been killed in clashes in several southern provinces as Saudi-led coalition aircraft targeted the Iran-backed insurgents, military officials said Saturday.

Fierce clashes raged in the areas of Al-Zaher and Thi al-Naem in Baida province between Huthi rebels and Popular Resistance fighters allied with forces loyal to President Abedrabbo Mansour Hadi, tribal sources said. The clashes overnight left 19 rebels and 14 Popular Resistance fighters dead, according to tribal and medical sources. Gun battles also flared up in Al-Madaribah in southwestern Yemen on the border between Lahj and Taez provinces, leaving an unknown number of fighters killed and wounded, military sources said.

Southern fighters said they were defending the area against rebels trying to reach the shores of the strategic Bab al-Mandab strait, some 50 kilometres (30 miles) away, which was seized by loyalists last month. The narrow waterway, which separates Yemen from Djibouti, funnels shipping to and from the Suez Canal in the northern Red Sea.

Witnesses said that many residents fled the area due to heavy fighting. Warplanes from a Saudi-led coalition raided positions of Huthis and allied renegade troops loyal to ousted president Ali Abdullah Saleh, military officials said. The air strikes destroyed rebel vehicles in the central Ibb province, close to Daleh, which was seized by pro-government forces along with four other southern provinces earlier this year. The coalition launched an air campaign against the rebels in late March, in support of Hadi who had been forced to flee to Riyadh. Around 5,000 people have been killed in the conflict since March, more than half of them civilians, according to UN estimates.

Moreover, the United Nations said on Saturday two contractors with the world body had been detained in Yemen, but gave no further details on the incident.

“Two contractors have been detained and DSS (Department of Safety and Security) is looking into it,” a U.N. spokesman said without elaborating or confirming if the two were American citizens.

A U.S. State Department official said it was aware of reports that two U.S. citizens had been detained at Sanaa airport in Yemen.

“The protection of U.S. citizens overseas is one of the Department of State’s highest priorities, including in these cases,” the official said in an email. “Due to privacy considerations, we do not have any further information to share.”

A Saudi-led Arab coalition intervened in Yemen’s civil war in March to try to restore the government after it was toppled by Iran-allied Houthi forces, but a mounting civilian death toll and dire humanitarian situation has alarmed human rights groups. The United Nations has designated Yemen as one of its highest-level humanitarian crises, alongside emergencies in South Sudan, Syria and Iraq. It says more than 21 million people in Yemen need help, or about 80 percent of the population.