Smoke billows following air strikes by the Saudi-led coalition on a weapons depot at a military airport, controlled by Houthi rebels, in Sanaa on Thursday. 

Reuters/Aden

A bomb exploded next to the governor's office in Aden and killed four people on Thursday, witnesses said, as Islamic State militants claimed responsibility for a deadly raid on a Yemeni military post near the Saudi border.

No one claimed responsibility for the blast in Aden, the temporary seat of Yemen's administration while the Houthi group holds the capital, Sanaa. An official said the governor was present but unhurt.

Hundreds of kilometres away in the Empty Quarter desert, a local official said unknown gunmen attacked a government military post in the Thamud district near Saudi Arabia, killing four soldiers.

In a statement posted online, Islamic State said it carried out the assault on the "apostate army," in the group's first known attack against the government since it emerged this year.

The attacks underscore the fragility of Yemen's government despite hundreds of air strikes from Gulf Arab states. That campaign has won the government control over most of the country and reversed the gains of the Houthis, an Iran-allied militia, after five months of civil war.

Plagued by chronic instability even before the latest civil war, Yemen is also home to the world's deadliest branch of al Qaeda and a new offshoot of Islamic State, which surfaced with a series of suicide bombings on Shia mosques on March 20 which killed 137 people.

On Thursday, Arab air strikes hit targets throughout northern Yemen, local officials said, as the front lines approach Houthi strongholds there.

The attacks hit five provinces controlled by the Houthis and the military airport in Sanaa amid rapid advances by Arab-backed Yemeni troops.

The Saudi-led alliance began its air strikes in late March as the Houthis entered Aden. The civil war has killed over 4,300 people and left diplomats and air groups appealing for a ceasefire to spare civilians and alleviate a mounting humanitarian disaster.

‘Senseless bloodshed’

One air raid on a school in Amran province north of Sanaa on Tuesday killed 13 teachers and four children, in an attack the UN called "senseless bloodshed."

"No nation, no society, can afford to lose its children to conflict - whether from direct attacks, from malnutrition, from disease, from lack of education, or from the traumas of the horrors they witness," UN agency Unicef said in a statement. It noted around eight children have been killed or wounded in the war every day.

Southern Yemeni militias and loyalist army units have scored major gains against the Houthis in the last month and may soon advance toward their stronghold around Sanaa.

But as a political accord remains elusive, suffering and hunger continue to spread, especially after the coalition bombing of northern Yemen's main entrepot in the Red Sea port of Hodaida this week.