The Saudi air force is attacking rebel strongholds in northern Yemen after rebels killed two Saudi security men on the border, the Saudi-owned, London-based Elaph website reported on Thursday.

Saudi government officials said only that the air force had bombed rebels who had seized a border area inside Saudi Arabia, which they said Saudi forces had now recaptured. The officials said at least 40 rebels had been killed in the fighting.

Al Jazeera televisison quoted a rebel spokesman as saying the Saudi air force had raided six locations inside Yemen, followed by a heavy attack on one position which had been hit by about 100 missiles in one hour.

There was no official confirmation from Riyadh or the Yemeni capital Sanaa of cross-border Saudi air strikes.

Saudi Arabia, the world's biggest oil exporter, has become increasingly anxious about instability and militancy in Yemen, which is also facing separatist sentiment in the south and a growing threat from Al Qaeda militancy.
   

An Al Qaeda group claimed responsibility on Thursday for the killing of seven Yemeni security officials in an ambush near the Saudi border this week.

Elaph, in an unsourced report monitored by the BBC, said Saudi ground forces were also moving towards the border area, but did not say any troops had crossed into Yemen.

The Shi'ite rebels, known as Houthis after the family of their leader, have previously accused Saudi Arabia of backing Yemen's armed forces in the conflict. Sanaa had denied this.

The Houthis said on their website late on Wednesday that Saudi planes had struck four locations using phosphorus bombs.
    Saudi Arabia said on Wednesday a security officer was killed and 11 were wounded in an attack by gunmen who had crossed the border from Yemen -- the first such reported incursion since the long-running Houthi revolt flared up again in August.

Elaph said another soldier had died later from the same clash, which it said occurred on Tuesday night near the village of Al-Khawbah in the Jizan region.

The rebels also said on Wednesday they had taken control of the Jabal Al-Dukhan area after defeating Saudi forces there.
   
Saudi Arabia was allowing the Yemeni army to use the mountainous area to launch attacks against them and they would take action if this continued, the rebels said.

Reporting the cross-border raid, the official Saudi news agency said gunmen fired on Saudi patrols in Jabal Al-Dukhan town, killing one security officer and wounding 11.
   

The 1,500 km (930 miles) border between Yemen and Saudi Arabia is a security worry for the kingdom, which is building a high-tech border fence to prevent infiltration.

Saudi Arabia and the United States fear the conflict in Yemen's north and a separatist movement in the south will enable Al Qaeda to establish a stronger foothold in Yemen.

Such fears were compounded in August, when a suicide bomber posing as a repentant Al Qaeda militant tried to kill the kingdom's counter-terrorism chief after coming from Yemen.

Arab countries allied to the United States, such as predominantly Sunni Muslim Saudi Arabia and Egypt, fear Shi'ite power Iran could gain influence in Yemen through the rebels.

Yemen's army launched Operation Scorched Earth in August to crush the rebellion. Aid groups say around 150,000 people have been displaced by the fighting, which first broke out in 2004.

The rebels say they are fighting political, economic and religious marginalisation by Yemen's central government. - Reuters