Yemenis gather at the site of a car bomb explosion near a mosque in Sanaa on Wednesday.

AFP/Sanaa

A car bomb claimed by the Islamic State group exploded outside a Shia mosque in Yemen's capital on Wednesday, killing four people, as loyalist fighters advanced on a rebel-held airbase in the south.

Witnesses said the blast was near a mosque of the Bohra sect of Shia Islam, not far from Ath-Thawra Hospital in the Rammah district of Sanaa, where several attacks claimed by IS have targeted Shia rebels who control the capital.

An IS Twitter user calling himself Karar al-Moayad said the jihadist group had carried out the attack and that it had left "several dead," while a medical source said another four people were wounded.

Four people were also killed in a similar explosion on July 21, according to a toll provided by the Houthi insurgents, who control large parts of Yemen.

Bomb attacks by the radical IS targeted several Shia mosques in the capital on March 21 and killed 142 people.

Meanwhile, clashes raged in the south, where pro-government forces expanded their area of control after recapturing the port of Aden, Yemen's second city, after four months of fighting.

The loyalists pushed back rebels in Lahoum, on Aden's northern outskirts, following heavy fighting in which 12 rebels were killed, military sources said.

Three pro-Hadi fighters were killed and dozens wounded, medical sources said.

The area is on the road to Lahj, where loyalists have been tightening the noose on rebels, aiming to recapture the strategic Al-Anad airbase.

The clashes and renewed anti-rebel air strikes on Wednesday by a Saudi-led Arab coalition highlighted the failure of a five-day truce declared from midnight on Sunday by the coalition to allow the delivery of desperately needed relief supplies.

Four months of fighting has left 3,984 people dead, nearly half of them civilians, according to the UN.