Israel introduced what it said was a new U.S. ceasefire plan and is trying to force Hamas to accept it by imposing a siege on the Gaza Strip.

The ceasefire’s first phase saw a surge in humanitarian aid after months of growing hunger. Hamas accused Israel of trying to derail the next phase Sunday hours after its first phase had ended and called Israel’s decision to cut off aid “a war crime and a blatant attack” on a truce that took a year of negotiations before taking hold in January.

Israel blocked aid trucks into Gaza as a cease-fire standoff has escalated, with Hamas calling on other mediators to intervene.