Jerusalem ? Palestinian militants fired three homemade rockets into southern Israel on Tuesday, threatening to unravel a cease-fire days after it began, and Israel responded by closing vital border crossings into Gaza.

Despite what it called a “gross violation” of the truce, Israel refrained from military action and said it would send an envoy soon to Egypt to work on the next stage of a broader cease-fire agreement: a prisoner swap that would bring home an Israeli soldier held by Hamas for more than two years.

Hamas, the militant Islamic group that rules Gaza, promised to rein in the Iran- and Syria-backed faction that carried out the rocket attacks and pledged to remain committed to the truce that went into effect June 19 and urged restraint by all sides.