SANAA — Yemeni Shiite rebels on Monday handed over the first of five Saudi soldiers held captive since their border war, resolving an issue that had complicated efforts to consolidate a four-day-old ceasefire with the Yemeni government.

"The Saudi soldier, Yahya Abdullah al-Khuzai, was handed over today (Monday) in the city of Saada to mediators," working on implementing the truce, rebel spokesman Mohammed Abdul Salam told AFP by telephone.

"This is a humanitarian gesture to ease tension. The other side (Saudis) should work now to end the prisoners issue," he said, adding that Khuzai had a leg injury.

A mediator confirmed the release and told AFP that efforts were under way to "release the other four captives within hours."

The mediator, who requested anonymity, said the rebels were "convinced" during lengthy talks "to leave the question of their prisoners taken by the Saudis, numbering 31, to the Yemeni authorities to follow up."

The rebels complained on Sunday that Saudi Arabia was refusing to release the prisoners it is holding in exchange for the five soldiers.

The truce, which came into force late on Thursday, continued to hold on Monday and joint commissions were meeting to implement its terms, both sides said.

Though the fighting has stopped, casualties in the north continue to mount.

Seven people -- two soldiers, three rebels and two civilians -- were killed Monday when an anti-tank mine exploded under a de-mining vehicle in Al-Iqab south of Saada city, local sources said.

On Sunday 13 Yemeni soldiers died when a military helicopter evacuating wounded servicemen crashed in the northern mountains, the defence ministry said on Monday.

US Secretary of State Hillary Clinton on Monday welcomed the ceasefire but expressed concern for the plight of the 250,000 people who fled the six years of fighting between the rebels and the Yemeni army.

"We understand that a mediation commission representing all parties is monitoring compliance with the terms of the ceasefire and beginning the urgent process of reconciliation and reconstruction needed to bring this conflict to a permanent end," she said.

"The United States remains concerned about the humanitarian situation in the area, including the approximately 250,000 Yemenis displaced by the fighting.

"To assist displaced Yemenis, the United States has thus far provided 19.3 million dollars during fiscal years 2009 and 2010. We urge other donor nations to support international relief agencies."

The six-point truce requires the rebels to reopen three major routes in the first stage: the road between Saada, Harf Sufian and the capital, Sanaa; the road from Saada west to Malahidh and the road from Saada east to Al-Jawf.

The rebels said on Monday that they have opened the road from Saada to Sanaa, along with two other roads.

The truce agreement also calls for a rebel withdrawal from government buildings, the return of arms seized from security forces, release of all prisoners including Saudis, handover of captured army posts, and a pledge not to attack Saudi Arabia.

The Saudis joined the fighting in November after accusing the rebels of killing a border guard and occupying two small villages.

Saudi ground troops and aircraft engaged the rebels in operations which the rebels said continued even after their fighters had withdrawn from all Saudi territory.

Yemen has accused the rebels of seeking to restore the Zaidi Shiite imamate that ruled in Sanaa until its overthrow in a 1962 republican coup that sparked eight years of civil war.

The rebels, who complain of economic and political discrimination against the north's Zaidis, have denied the charge.

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