At least 40 people are confirmed dead and 150 injured after both sides exchanged fire shortly after agreeing to an unconditional humanitarian cease-fire beginning at 1 a.m. ET on Friday.
Israel says the latest round of fighting comes after Hamas launched eight rockets and mortars, all either intercepted or landing in unpopulated areas, prompting Israel Defense Forces launch air strikes on the city of Rafah near the Israel/Egypt border and resume full operations in the Gaza Strip.
"If corroborated, this would constitute a serious violation of the humanitarian ceasefire in place since 8 a.m. this morning by Gazan militant factions, which should be condemned in the strongest terms," United Nations special coordinator Robert Serry said in a statement.
"Mr. Serry urges the Palestinian parties to last night's understanding to urgently reaffirm their commitment to the humanitarian cease-fire. He is deeply concerned regarding the serious consequences on the ground that could arise as a result of this incident. He will continue his efforts to contain the violence and the risk of renewed escalation."
IDF forces are also searching for a missing Israeli solider, believed to be captured by Hamas militants.