Earlier on Tuesday, Mohammed Abd al-Salam, a spokesman for the Houthi group, had said the prisoners would be freed within 48 hours and that he expected government-held prisoners would also be released.

Doubtable truce

Analysts have said the truce agreement between the government and rebels, who belong to the minority Zaidi sect of Shia Islam, was unlikely to last as it does not address the rebels' complaints of discrimination by the goverment.

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Abd al-Salam said "this issue, once resolved, will serve the cause of peace," adding that the rebels have not dragged their feet in implementing the terms of the ceasefire.

He also said the rebels object to the army leaving its barracks in the north, but that they have no objection to civil servants returning to their posts in the region.

On Tuesday, the Sanaa government said that the Houthis were failing to meet some of the six key points of the ceasefire agreement.

"The Houthis continue to hamper the work of the committees" charged with implementing the ceasefire, the official Saba news agency quoted a spokesman for the high security commission as saying.

He accused the group of "procrastination," and said that the rebels have refused to turn over cleared mines to authorities, have retaken control of some areas soon after withdrawing, and have also created new checkpoints.

The security commission spokesman urged the rebels to "fully comply with the terms of the ceasefire" to "normalise the situation and re-establish peace" in the north, particularly in the rebel stronghold of Saada province.

Truce terms

Under the truce terms the rebels should free all prisoners, open roads in the north, withdraw from government buildings, return weapons seized from security forces and hand over captured army posts.

They also had to pledge not to attack Saudi Arabia.

The Houthi rebels have engaged in sporadic clashes with government forces since 2004, but the situation had turned particularly violent after government forces launched "Operation Scorched Earth" against the rebels last August.

Saudi Arabia joined the fray in November after accusing the Houthis of killing a border guard and occupying two villages inside Saudi territory.

Western governments and neighbouring Saudi Arabia, the world's biggest oil exporter, fear al-Qaeda is exploiting instability on many fronts in Yemen to recruit and train fighters for attacks in the region and beyond.