Russian forces have launched a counteroffensive in eastern Ukraine, Ukrainian military officials said on Monday, as fighting continued in the region despite a cease-fire agreement reached in July. The Ukrainian military reported intense shelling and clashes in the Donetsk and Luhansk regions, where pro-Russian separatists have been fighting against Ukrainian forces since 2014. The Ukrainian military said it was responding with artillery and mortar fire. The conflict, which has killed more than 13,000 people, flared up again in late August, after a lull in fighting that followed the cease-fire agreement.