Casualties have been reported in Saudi Arabia following attacks along its border with Yemen, forcing Riyadh to suspend all flights in one of its southern airports, and shut down all schools. 

Saudi authorities said that armed Houthi fighters carried out the attacks on Tuesday in the town of Najran.

The Associated Press news agency reported that at least two civilians were killed, and five soldiers were captured in the attack.

Shelling at the border has also been blamed for the suspension of all flights of the kingdom's national carrier, Saudia, to and from Najran, showing the insurgents' ability to launch assaults despite weeks of Saudi-led airstrikes targeting them. 

Yemeni tribal leaders told the Associated Press news agency that armed rebel Houthis, shelled the border area from inside Yemen. The sources spoke on condition of anonymity as they were not authorised to speak to the media.

The border area has been a flashpoint for Saudi-Houthi clashes amid a Saudi-led coalition's air campaign targeting the rebels in Yemen.

"This is a major development in this war," Al Jazeera's Mohamed Vall, who is reporting from the Saudi capital Riyadh, said.

"Up until now we have seen attacks on soldiers near the border, but not in the city itself."

He said the latest attack complicates the work of the Saudi military, who now faces the challenge of securing the border, while carrying out air strikes inside Yemen.  

Last Thursday, border clashes left three Saudi troops and dozens of rebels dead.

News of the latest violence comes as Saudi King Salman announced at a meeting of Gulf leaders the establishment of a centre to coordinate relief operation for Yemen, and invited the United Nations to join in the effort.

"We hope that the United Nations will participate effectively with what this centre will shoulder, including coordinating all humanitarian and relief works for the Yemeni people with the participation of the countries that are supporting the Gulf initiative," he said in a speech in the Saudi capital.

On Monday, Saudi's foreign minister Adel-al Jubeir had said that his country was considering temporary halts in coalition air strikes against the Houthis, to allow for aid deliveries.

For his part, John Kerry, US secretary of state, will visit Riyadh for discussions with Saudi government leaders on May 6 and May 7 to discuss the "humanitarian pause".

The UN has repeatedly warned that impoverished Yemen faces a major humanitarian crisis and calls have been growing for efforts to increase aid deliveries.

Jubeir said Saudi Arabia "plans to establish a centre on its territory to be in charge of coordinating all humanitarian aid efforts" with the UN, donors and other relevant agencies.

He warned the rebels against "taking advantage" of any pause in the bombing.

Saudi Arabia "will deal with any violations in connection with the suspension of air strikes or movements that hinder humanitarian efforts", he said.

Fuel for infrastructure

The UN has called for a humanitarian pause in the conflict, as relief agencies said they desperately need supplies, including fuel to run infrastructure such as hospitals.

It warned that key infrastructure in the war-torn country, including water supplies, health services and telecommunications, are on the verge of breaking down due to a major fuel shortage.

The UN's Van Der Klaauw, said on Saturday that an arms embargo was affecting delivery of supplies, urging a humanitarian pause "at least for a couple of days".

At least 1,200 people have been killed in fighting in Yemen since March 19 and thousands more have been wounded, according to the UN. It estimates that at least 300,000 people have been displaced by the conflict.

Meanwhile, residents in Yemen's war-torn city of Aden said fighting there has displaced hundreds more people as rebels press their offensive to capture large sections of the southern port city.

The residents, after being trapped inside their homes for weeks in the Tawahi district, said they were fleeing after Houthi rebels shelled and stormed the area, along with the military units loyal to ousted President Ali Abdullah Saleh.