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 ce…