LONDON (AlertNet) - Already overcrowded aid camps in Yemen are being stretched further as hundreds of people arrive daily as they flee escalating fighting between government forces and Shi'ite rebels that has dragged in neighbour Saudi Arabia, aid organisations said.

Saudi Arabia 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.

The world's largest oil exporter has used its air force and artillery to try and push the rebels beyond a 10km (6 mile) buffer zone from its border -- further worrying aid groups already concerned about the crisis in Yemen, the Arab world's poorest country.

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 on Friday at the escalation of the conflict in the north, where the United Nations now says 175,000 people have been displaced by the fighting, up from around 150,000 a few weeks ago.

"The extension of the conflict along the border is concerning us because civilians are paying a high price," Dorothea Krimitsas, a spokeswoman for the International Committee of the Red Cross (ICRC) in Geneva, said.

Most of the fighting is around the town of Sa'ada, where the ICRC and the Yemen Red Crescent have three more camps. Altogether some 11,000 people are living in their camps.

CAMP POPULATIONS GROWING

An ICRC statement said that in the village of Mandaba, close to the Saudi border, up to 280 people were arriving every day for help. The ICRC and the Red Crescent have set up a camp there, which has swelled to 6,000 people from 2,500 only one month ago. 

The fighting had become so intense around Sa'ada that not all the people who need help could be reached, the ICRC said. Only one of the three main hospitas in and around the town was fully operational, it added.   "Testimonies from people who have reached us have made clear how bad it was," Krimitsas said. "People have been trapped for weeks without access to medical care."

Sigrid Kaag, the regional head for the Middle East of the U.N.'s children fund, UNICEF, said she was "deeply concerned" about the escalation of the conflict.

"Fighting has now spilled into Saudi Arabia, reportedly causing 240 villages to be evacuated and more than 50 schools to be closed," Kaag said in a statement.

Overcrowding has become a major worry. The population of the al-Mazraq camp, one of the biggest, has doubled in four weeks to 15,000 people with another 28,000 people living on the camp's perimeters, UNICEF said.

"Deaths have been recorded among children in the camp as malnutrition, already a chronic problem in Yemen, is reaching alarming levels," Kaag added.

More than 600 children in the camp are being treated for severe acute malnutrition, she said. 

CAPACITY EXCEEDED

Andrej Mahecic, a spokesman for the U.N.'s refugee agency UNHCR, said the volume of people was becoming a major concern and that around 900 people were arriving everyday at the al-Mazraq camp, which had exceeded its capacity.   "Three or four families now share a tent normally meant for one," he said.   A U.N. appeal for $23.75 million has so far raised nearly half the cash.   Getting aid into the area around Sa'ada has been a problem.

The UN had been using an aid corridor from Saudi Arabia until the fighting around the border made this too dangerous.

It said it hoped fighting in the area would subside and allow it to resume the aid supply missions.

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