By Ulf Laessing

ALB, Saudi Arabia, Oct 11 (Reuters) - A United Nations aid convoy, delayed by fighting between Shi'ite rebels and government forces in northern Yemen, crossed into the country from Saudi Arabia on Sunday.

The aid was delivered to refugees from a new wave of fighting between the army and insurgents of the Zaydi Shi'ite Muslim sect, in which hundreds of people have been killed and tens of thousands displaced.

"We hope to have more convoys," said Sultan Khilji, the convoy's U.N. protection officer, adding that up to 5,000 people were stranded in the northern border region.

The three-truck convoy carried 200 tents, blankets and mattresses to about 300 refugees stranded in the Yemeni province of Saada which has seen heavy fighting since the army launched a major offensive on Aug. 11.

The U.N. refugee agency said on Friday it would send aid to northern Yemen from Saudi Arabia for the first time but the convoy had been delayed for security clearance.

It crossed the frontier at Alb, about 20 km (12 miles) north of the area of Baqim, the scene of fierce clashes.

Aid agencies are warning of a humanitarian crisis in northern Yemen, where up to an estimated 150,000 people have fled their homes since Shi'ite tribesmen launched the insurgency in 2004. Limited access to the war zone means they have no clear idea of exactly how many have fled.

The northern Zaydi rebels say they suffer religious discrimination by Sunni hardliners who have gained ground due to President Ali Abdullah Saleh's close ties to Saudi Arabia which adheres to a puritanical form of Sunni Islam.

The United States and Saudi Arabia fear that the fighting in northern Yemen, and street clashes with separatists in the south, could create instability that al Qaeda could exploit to attack Saudi Arabia.

Zaydis make up about a third of Yemen's population of about 23 million people. The majority of Yemenis are Sunni Muslims.

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