UN Secretary-General Antonio Guterres on Sunday warned that the situation in Gaza Strip is declining rapidly, repeating the appeals for a ceasefire to end the "nightmare" of bloodshed.

"The situation in Gaza is growing more desperate by the hour. I regret that instead of a critically needed humanitarian pause, supported by the international community, Israel has intensified its military operations," Guterres said.

He added that the number of civilians who have been killed and injured is totally unacceptable.

"The world is witnessing a humanitarian catastrophe taking place before our eyes," Guterres said.

"More than two million people, with nowhere safe to go, are being denied the essentials for life –- food, water, shelter and medical care –- while being subjected to relentless bombardment. I urge all those with responsibility to step back from the brink," he said.