The Islamic group Hamas has announced it will delay the release of hostages scheduled for Saturday as it accuses Israel of violating the six-week ceasefire agreement.

Hamas said in a statement on Monday that the Israeli military attacked residents in the Gaza Strip, and that Israel failed to fully allow aid supplies to enter the enclave.

The group said the sixth round of hostage release scheduled for Saturday will be postponed. Three hostages were reportedly expected to be freed.

In response, Israeli Defense Minister Israel Katz said that Hamas' announcement violated the ceasefire deal, and that he had instructed the military to be at the highest level of readiness in Gaza and for domestic defense.

Under the agreement that took effect on January 19, Hamas has released a total of 16 Israeli hostages, and Israel has set free more than 760 Palestinians.

The Wall Street Journal says the postponement is the "most serious challenge yet to a fragile ceasefire" under which another 17 Israeli hostages are scheduled to be returned to Israel over the coming weeks.