A Hamas official says Israel’s delay in the release of some 600 Palestinian prisoners is a “serious violation” of the ceasefire agreement and talks on a second phase of the accord are not possible until they are returned.

The release of 620 Palestinian prisoners was delayed for several hours and was meant to occur after six Israeli hostages were released on Saturday.

The six Israelis were the last living captives set to be released in the first phase of the cease-fire. The agreement, strained for weeks, has an uncertain future.