Sana’a: Seven pro-government activists were heading to Yemen’s main southern city of Aden on Monday as part of a prisoner swap overseen by the International Committee of the Red Cross, rebel officials said.

The seven activists belong to the Southern Movement, whose fighters were locked in deadly battles with Al Houthi militiamen, who they helped to push out of Aden in mid-July.

They will be exchanged for seven Al Houthi militiamen, the officials said.

The activists will depart with ICRC head Peter Maurer from the militia-held capital Sana’a aboard a Red Cross plane, according to a journalist who saw the prisoners at the airport.

Aden was the scene of deadly battles between local loyalist forces and rebels who entered in March, days before a Saudi-led coalition launched an air war against the Al Houthis and their allies across Yemen.

Mauer arrived in Sana’a on Saturday at the start of a three-day visit to assess the “dire humanitarian situation” in the country.

The United Nations says nearly 4,000 people have been killed since March, half of them civilians, while 80 per cent of Yemen’s 21 million people are in need of aid and protection.

The ICRC says 1.3 million Yemenis have been displaced by the conflict.

Meanwhile, fighters opposed to the Al Houthis seized four districts in the central province of Ibb on Monday, residents and local officials said, bringing the armed resistance closer to the group’s stronghold in the capital Sana’a.

Tribal gunmen and Islamist militias loyal to Yemen’s exiled government took control of the areas amid heavy clashes with the Al Houthis, in the latest of a series of northward gains with the backing of Gulf Arab air strikes and weapons.

The northernmost district overrun, Al Radma, is 125km from Sana’a, which was conquered by the Iran-allied Al Houthis in September in what they called a revolution against corrupt officials backed by the West.

Deadlocked for almost four months, the war has tipped somewhat to the advantage of the Al Houthis’ opponents, with their seizure of Aden last month and advance into nearby areas with the help of tanks and heavy artillery shipped by the UAE.

The southern fighters battled Al Houthi forces on Monday in the southern city of Lawdar, one of the last Al Houthi strongholds in Abyan province.