Some 240 villages in Saudi Arabia have been evacuated and scores of schools closed due to fighting which has now spilled over from Yemen, the United Nations Children's Fund (Unicef) said on Friday, citing local contacts.

Saudi Arabia, the world's biggest oil exporter, launched an offensive last week after Yemeni rebels seized Saudi territory along the mountainous border from which they said the Saudis had been allowing Yemeni troops to use to attack their positions.

'Fighting has now spilled into Saudi Arabia, reportedly causing 240 villages to be evacuated and more than 50 schools to be closed,' Sigrid Kaag, Unicef's regional director for the Middle East and North Africa, said in a statement.

The information came from Unicef's contacts on the ground, a spokeswoman said in Geneva, giving no further details.

A Saudi government adviser said on Thursday that Saudi Arabia is using air power and artillery to enforce a 10 km deep buffer zone inside Yemen to keep the Shi'ite rebels away from its southwestern border.

Fighting between Yemeni troops and Houthi rebels, who say Yemen's Zaidi Shi'ite minority suffers discrimination and neglect, has flared on and off since 2004 in the northern province of Saada.

Unicef voiced deep concern at the escalation of the conflict in north Yemen, where the United Nations now says 175,000 people have been displaced by the fighting.

More than 15,000 are staying in al-Mazraq camp in Hajjah province, the population of which has doubled in the past month, according to the UN children's agency.

'Deaths have been recorded among children in the camp as malnutrition, already a chronic problem in Yemen, is reaching alarming levels,' Kaag said. More than 600 children in the camp are being treated for severe acute malnutrition, she said.

Sudden influx

The UN refugee agency said that up to 900 people have been arriving every day at al-Mazraq which has exceeded its capacity. UNHCR estimated its current population at 10,000.

'The latest sudden influx is adding more pressure on an already dire situation, and the overcrowding in the camp is becoming a major concern,' UNHCR spokesman Andrej Mahecic said. 'Three or four families now share a tent normally meant for one.'

The International Committee of the Red Cross (ICRC) voiced concern at the worsening plight of civilians as the conflict intensifies.

The humanitarian agency called on all sides in the conflict to spare civilians and allow the safe, unimpeded passage of aid.

'All persons detained in connection with the conflict must be treated humanely. And we stand ready to visit any such persons,' ICRC spokeswoman Dorothea Krimitsas said.

Due to the steady flow of refugees, the ICRC and Yemeni Red Crescent set up a camp at Mandaba in Baqim, close to the Saudi border in northwestern Yemen, where it is providing food rations and clean water to some 6,000 people, she said.

It is increasingly difficult for civilians to reach medical and health facilities in northern Yemen, she added. In Saada city and nearby areas, only one of three main hospitals is fully functional, according to the ICRC.-Reuters