JABALIA, Gaza Strip—The United Nations blamed Israel for the second deadly assault in a week on a shelter for displaced Gazans and warned of a dire humanitarian crisis with many in the Palestinian territory already lacking electricity, potable water or safe refuge.

The strike on the U.N.-run shelter early Wednesday killed 15 people, according to Gaza health ministry. Hours later, Israel declared a brief, unilateral pause in the bombardment. As Palestinians used the four-hour window to stock up on provisions, explosions rocked...