GAZA CITY — The head of Hamas, the militant Islamic group that rules the Gaza Strip, accused Israel on Thursday of “blackmail” and slow-walking the easing of border restrictions under a tenuous cease-fire agreement reached last month after repeated rounds of violence.

“The understandings today are in critical condition,” the Hamas leader, Ismail Haniya, told foreign journalists at a rare news conference here in Gaza City.

The hot-and-hotter Israel-Gaza border nearly devolved into a full-blown war during a bloody weekend of rocket attacks and airstrikes in May, before both sides were pulled back from the brink by Egypt and the United Nations, which brokered a standoff agreement through indirect talks.

Complicating matters: The terms of the cease-fire are not written and made public, leaving each side to fault the other, with little public accountability.