He called on Israel to restore fuel supplies to Gaza, and to allow the passage of humanitarian assistance and commercial supplies, sufficient to allow the functioning of all basic services and for Palestinians to live their daily lives.
"The collective punishment of the population of Gaza, which has been instituted for months now, has failed," he said. "The immediate and common goal must be an end to violence and a reopening of crossings.
John Ging, UNWRA chief in Gaza, said Israel must supply gasoline to Gaza immediately or the UN won't be able to distribute desperately needed aid packages to Gaza residents.
According to Ging, the agency's supply of gasoline will run out Thursday. Some 860,000 of Gaza's 1.4 million people receive aid from UNWRA, and an additional 270,000 are serviced by the UN's World Food Program.
Serry noted that the situation in the Strip was unsustainable due to "the killing of civilians, including many children here in Gaza," among other things.
"We are at a crossroads and the parties must make responsible choices to step back from the brink. UN Secretary-General Ban Ki-moon is extremely concerned at the humanitarian, human rights, security and political crisis here," Serry said. " He and I and the entire UN system are absolutely committed to averting a further worsening of the situation, and seeing Gaza return to better times."
'Attacks on crossing disturbing'According to Serry "the UN is leading the humanitarian effort to sustain the people of Gaza under conditions of great adversity. We are also very active, politically and diplomatically, pushing all parties, and the international community, to work for a different and more positive strategy for Gaza."
"We are giving our strong support to the current Egyptian efforts to calm the violence, and we call on all concerned to work with Egypt in that effort," he said.
The UN envoy also condemned the recent Palestinian attacks against crossing points into Gaza, saying they were "deeply disturbing."
"I appeal to Hamas to immediately end attacks against the crossings, whether by it or any other faction or group. These attacks endanger both international and Israeli civilians, and cannot possibly contribute to Palestinian efforts to ease the blockade of Gaza. On the contrary, they serve only to deepen and prolong it," he said.
AP contributed to this report