Houthi forces fired mortar rounds on Monday at an oil refinery in the southern Yemeni city of Aden, hitting three full storage tanks and starting a huge fire, witnesses and local officials said. The Houthis, who took over the capital, Sana, last September, have been battling local fighters in the south for three months and had attacked the refinery before. An Arab coalition led by Saudi Arabia has been bombing the Houthis to try to restore Yemen’s exiled government, and it has imposed a near-blockade on the country, cutting off trade in food and fuel. A weeklong United Nations-brokered cease-fire was meant to begin Saturday to allow the delivery of aid, but it failed to take hold after Saudi Arabia refused to recognize the truce and continued airstrikes.