At least eight army medics have been killed and nine others injured while riding a bus in southern Yemen after gunmen opened fire from the roadside, officials told Al Jazeera.

The attack, in which the armed men used Kalashnikov assault rifles, happened on Saturday morning about 15kms north of Aden while it was en route to the nearby Basaheeb military hospital.

I haven't seen anything scarier than this. The gunmen came out suddenly and opened fire

Mohammed Saleh, witness at the scene

Some of the dead were working for Basaheeb hospital while others were employed at the Fourth Military District, the correspondent said.

Two woman were among the dead, local security officials told the AP news agency.

"I haven't seen anything scarier than this. The gunmen came out suddenly and opened fire," said Mohammed Saleh, a civilian witness at the scene, told Reuters news agency.

Al Jazeera's Saeed Thabet, reporting from the Yemeni capital, said the gunmen launched their attack as soon as the driver slowed down ahead of a speed bump.

No one has claimed responsibility for the attack.

"It could be an act of reprisal by al-Qaeda group just two days after a US drone strike killed five suspected al-Qaeda fighters in the southern Mafraq al-Saeed area in Shabwa province," Thabet said.

Yemen is the main stronghold of al-Qaeda in the Arabian Peninsula (AQAP), one of the armed group's most active wings.

The US acknowledges using drones to combat AQAP in Yemen but does not comment publicly on attacks.

Attacks against Yemen's security forces have increased since the army launched an unprecedented campaign beginning in late April to dislodge al-Qaeda fighters from strongholds in southern Yemen.

Al-Qaeda has carried out several hit-and-run attacks since the Yemeni army drove it out of its strongholds of Shabwa and Abyan last month.

Source: Al Jazeera and agencies