Air raid sirens blared over Ukraine's capital on Wednesday as officials said they were bolstering defenses in key cities threatened by Russian forces. Thousands of people are thought to have been killed, both civilians and soldiers, in almost two weeks of fighting since President Vladimir Putin's forces invaded. While Russian troops have seen their advance slowed by fiercer than expected Ukrainian resistance, they have laid siege to several cities, trapping civilians inside them with little or no food, water or medicine. Repeated efforts to establish safe evacuation routes out of several urban areas have failed, though a few thousand people managed to flee the northeastern city of Sumy via a safe corridor on Tuesday. Residents of the encircled Azov Sea port of Mariupol were not so lucky: Some of the worst desperation of the war is unfolding there, but an attempt to evacuate civilians and deliver badly needed supplies failed, with Ukrainian officials saying Russian forces had fired on the convoy before it reached the city. Ukrainian authorities announced Russia has agreed to a new daylong cease-fire along several evacuation routes for civilians fleeing besieged or occupied cities Wednesday, though it is unclear whether Russian forces will respect it. Meanwhile, Ukraine's general staff of the armed forces said in a statement that it was building up defenses in cities in the north, south and east, and that forces around Kyiv, the capital, were resisting the Russian offensive with unspecified strikes and “holding the line.”