Huge plumes of smoke could be seen rising above Jebel al-Dukhan, a 2,000-metre (6,600-foot) peak that marks the frontier near the border town of Al-Khubah, some 60 kilometres (40 miles) inland from the Red Sea coast.

Saudi commanders said troops were shelling suspected rebel positions.

At the district hospital in the nearby town of Samtah, a medical official said seven Saudis, four of them women civilians, had been killed and 126 people wounded since the fighting first erupted on Tuesday.

The official said Friday had been the "heaviest day" for casualties so far, with two soldiers killed and many people wounded.

Saudi media said the four women, all of them from the same family, had been killed in rebel shelling.

They said fighting flared again on Friday night after rebels dress as women infiltrated the Saudi border villages of Al-Qarn, Qawa and Al-Dafeneyah.

Local news website Jazannews.com said some 40 rebels had surrendered to Saudi forces.

Saudi authorities say the fighting erupted when the rebels attacked a border post, killing one Saudi border guard and wounding 11.

But the rebels said they had come under attack by Saudi ground troops inside Yemen. They said they had repulsed the cross-border assault, capturing a number of soldiers.

"With Allah′s help, the Saudi tyrannical advance into Yemen′s territory has been defeated," a statement on the rebels′ website said.

"A number of its troops have been captured and several military vehicles and supplies been seized."

A Saudi statement said the air force had struck the border zone on Wednesday and Thursday to clear areas where the rebels had encroached on the kingdom′s territory.

But a Saudi government adviser told AFP on Thursday that F-15 and Tornado jets had bombed rebel camps inside Yemen with Sanaa′s blessing.

The adviser′s comments were the first acknowledgement of Saudi involvement in the Yemeni government′s efforts to put an end to the five-year-old uprising by Zaidi Shiite rebels in the mountainous north of the country, one of the world′s poorest.

Saudi authorities have evacuated residents of border villages to newly erected camps.

In Yemen, relief agencies have expressed mounting concern about getting aid to the tens of thousands of people who have fled the fighting since the government launched an all-out offensive on August 11.

Washington has voiced concern over the expansion of the conflict to involve its key Gulf ally.

"It′s our view that there can be no long-term military solution to the conflict between the Yemeni government and the rebels," State Department spokesman Ian Kelly said on Thursday.

But Bahrain, Kuwait and the United Arab Emirates all voiced support for the regional kingpin in its battle with the Yemeni rebels.

Yemeni authorities accuse the rebels of seeking to restore the Zaidi imamate that ruled in Sanaa until its overthrow in a republican coup in 1962 that sparked eight years of civil war. The rebels deny the charge.

An offshoot of Shiite Islam, the Zaidis form the majority community in the far north but are a minority in mainly Sunni Yemen.

Yemeni President Ali Abdullah Saleh is himself a Zaidi.