At least 12 civilians have been killed after Saudi warplanes attacked a pick-up truck in the northwestern Yemeni province of Hajjah on Wednesday, residents say.

Locals said the passengers were shoppers heading to a local market in the Hiran area, which is controlled by the Houthi movement, adding six others were injured.

Separately, Saudi aircraft carried out bombings in the Nihm district in Sana’a province and Dhubab district in Ta'izz province, but there was no immediate word on possible casualties or damage.

Meanwhile, Yemen's army snipers killed four Saudi troops in Rabou’ah in Saudi Arabia's southwestern region of Asir, media reports said.

Thousands of Saudi airstrikes have failed to dislodge the Houthis from the capital, Sana'a, but have hit schools, markets, hospitals and homes, killing many civilians which rights groups say may amount to war crimes.

More than 11,000 people have been killed in the 20-month Saudi war which has unleashed a humanitarian crisis in one of the Arab world's poorest countries.

'Campaign of harassment'

On Wednesday, Amnesty International said Saudi-backed militants fighting in Yemen are leading a "campaign of harassment and intimidation" against hospital staff and using civilians as human shields.

Advocacy Director for the Middle East and North Africa at Amnesty International Philip Luther said the Saudi mercenaries are stationing fighters and military positions near medical facilities.

"By positioning fighters and military positions near medical facilities they have compromised the safety of hospitals and flouted their obligation to protect civilians under international law," he said.

Luther said the Saudi-backed militants are also harassing medical staff and preventing doctors from carrying their work.

"Attacks targeting health professionals or medical facilities are prohibited by international humanitarian law and can constitute war crimes," he said.

Saudi Arabia is waging a destructive war in Yemen in order to restore Abd Rabbuh Mansur Hadi to power. Hadi resigned as president and fled the capital Sana’a to Riyadh in March last year. He returned to Aden in September.

Saudi Arabia and its mercenaries have already been accused of gross human rights violations in Yemen, including repeatedly attacking hospitals and civilians.

In its Wednesday report, Amnesty cited cases in which hospitals were shut down because of threats against staff by Saudi-backed forces.

In one incident on 21 November, the militants closed the biggest medical facility of al-Thawra in Tai'zz apparently after hospital staff treated three injured Houthi fighters.

"According to eyewitnesses three armed men stormed an office at the hospital and threatened to kill medical staff if it was not shut down immediately," the rights group said.

Loyalists also tried to drag two surviving Houthi fighters -- one a minor -- out of intensive care and recovery units, Amnesty said, calling it "outrageous and unacceptable."

The report cited 15 doctors who described abuses by the pro-Hadi militants, including requesting preferential medical treatment, insulting the staff and diverting electricity for personal use.

"Doctors told Amnesty International that if anti-Houthi fighters were turned away due to lack of capacity at the hospital in some cases they turned violent or abusive," the organization said.

"In other cases, medical staff said that doctors were forced to carry out their work at gunpoint," it added.

According to one doctor, one militant opened fire inside the al-Jamhouri hospital’s compound after being told his son did not require emergency care and could be treated by a nurse. The gunfire injured hospital staff and killed a patient.

Amnesty quoted an administrative worker as saying that pro-Hadi militants had threatened medical staff and interfered with the hospital’s administration and its decision-making "hundreds of times".

"When we stand up to them, they threaten us with being killed," the unnamed worker said.