DEIR AL BALAH, Gaza Strip — Aid groups are raising new alarm over Israel’s blockade of the war-ravaged Gaza Strip, where it has barred entry of all food and other goods for more than six weeks. Thousands of children have become malnourished, and most people are barely eating one meal a day as stocks dwindle, the United Nations says.

U.N. Secretary General António Guterres, in some of his strongest language on the crisis so far, described Gaza as “a killing field.” Israel accused him of “spreading slander.”