A suicide bomber suspected of belonging to Al-Qaeda killed five Yemeni soldiers when he detonated his explosive belt on Tuesday among army recruits in Aden, a security official said.

The man mixed in with a group of recruits as they headed to their base in the southern city which serves as a temporary capital for the government, the official said.

Army and government installations have been the target of several attacks by extremists since pro-government forces drove out Shia rebels in the summer.

Al-Qaeda and the Islamic State group have taken advantage of the chaos caused by the war between the government and the rebels to strengthen their grip on southern Yemen.

But after having long ignored them, forces loyal to President Abedrabbo Mansour Hadi have launched operations against them in the past few weeks, backed by the firepower of a Saudi-led military coalition.

Meanwhile, loyalists and rebel fighters have clashed on several fronts in Yemen, officials said on Tuesday, the second day of a UN-brokered ceasefire the insurgents have warned is in jeopardy.

Forces loyal to President Abedrabbo Mansour Hadi fought with Houthi rebels in the province of Marib, east and north of the rebel-held capital Sanaa, officials said.

The rebels and their allies loyal to ousted president Ali Abdullah Saleh advanced overnight in the area of Sarwah, in Marib, wresting control of two hills, a military official said.

The Saudi-led coalition, which launched a military campaign against the rebels last year, had described violations on Monday as "minor".

Seven soldiers have been killed and 15 others wounded in Sarwah in rebel attacks since the ceasefire entered into force at midnight between Sunday and Monday, the official said.

There were also clashes in Nihm, northeast of Sanaa, military officials said, while rebels said they confronted an attack by Hadi's forces in the area.