"The number of Palestinians killed in Gaza so far is nine and the number is increasing," the envoy, Riyad Mansour, told reporters. Palestinian sources in Gaza said seven had been killed, including a top Hamas military commander.
"The situation is very explosive," Mansour said.
"We condemn in the strongest possible terms this latest aggression against our people," he said, adding there was "no justification whatsoever" for the killing of Palestinians by Israeli forces.
"They are mobilizing a large number of forces, ground forces, with the possibility of moving in the Gaza Strip," the UN envoy said.
Egypt said it was recalling its ambassador over Israel's hit on a car in Gaza City that killed Ahmed Jaabari and his bodyguard, swiftly followed by more than 20 air strikes in Gaza. Dozens of people were wounded in the raid.
Israel warned it was only the start of an operation targeting militant groups in Gaza, which comes as the Jewish state prepares for general elections in January.