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The United Nations on Friday urged Yemen's warring parties to exercise "restraint" after they traded accusations of violations of a UN-brokered ceasefire.

"I am following very closely the latest developments in #Marib and urge all parties to show restraint and their continued commitment to the truce as promised to Yemenis," tweeted Hans Grundberg, the UN special envoy for Yemen.

A Yemeni military source told AFP that loyalist forces had "repelled a Houthi (rebel) attack" in southern Marib, the government's last stronghold in the north of the country.

"The Houthis are taking advantage of the ceasefire to strengthen their military presence" around the strategic city, said the source, speaking on condition of anonymity.

On Tuesday, the government accused the Iran-backed rebels of endangering the truce with "military deployments, the mobilisation of troops and vehicles, and artillery fire and drone attacks".

For their part, the Houthis said Friday on rebel-run Al-Masirah television that they had "repelled an advance" by loyalist forces.

The government has been locked in conflict with the Houthis, who control the capital Sanaa and most of the north, despite a Saudi-led military intervention launched in 2015.

A fragile truce since last Saturday -- the first day of the Muslim holy month of Ramadan -- has largely held, offering a glimmer of hope in the conflict which has triggered what the UN calls the world's worst humanitarian crisis.

The brutal war has killed hundreds of thousands and left millions on the brink of famine in what has long been the Arab world's poorest country.