"The continuing threat of annexation by Israel of parts of the West Bank would constitute a most serious violation of international law, deal a devastating blow to the two-state solution, close the door to a renewal of negotiations, and threaten efforts to advance regional peace and our broader efforts to maintain international peace and security," Mladenov told the Security Council.
Recent polls show that the Israeli public is also divided on the issue. Countries in the region and many in the international community have clearly stated their positions regarding annexation, he said in a briefing.
The Palestinian leadership announced on Tuesday that it sees itself absolved "as of today, of all the agreements and understandings with the American and Israeli governments and of all the obligations based on these understandings and agreements, including the security ones" and calls on Israel to assume its obligations as the occupying power. The statement by President Mahmoud Abbas also calls for negotiations under international auspices, including by the Quartet, to advance a negotiated two-state solution, said Mladenov.
"Tomorrow I will be meeting with the Palestinian prime minister in order to better understand the practical side of the decision by the leadership and its implications on the ground. If there is one concern that we all share, that is the need for all to work together to prevent escalation and radicalization."
Mladenov does not see the Palestinian announcement as a threat.
"If I may speak openly and very frankly on the issue, whatever our individual assessments of the Palestinian reaction to the Israeli threat of annexation may be, it is certainly one thing -- it is a desperate cry for help. It is a call for immediate action. It is a cry for help from a generation of a leadership that has invested its life in building institutions and preparing for statehood for over a quarter of a century."
The Palestinian leadership is not threatening, it is calling for urgent action to preserve the prospect of peace, he said.
The Palestinian leadership is doing so at a time when a new, younger generation comes forward, with its own aspirations for the future, many feel betrayed and increasingly cynical, he said.
"We don't know what future they will carve out for themselves, but whatever future young Palestinians and Israelis decide to build, we have an obligation to prevent violence and protect the chance for peace. Therefore, I ask you today, to join the secretary-general in his call against unilateral steps that will hinder current diplomatic efforts to create the conditions for bringing Israelis and Palestinians back to the table."
Mladenov asked the other parties of Middle East Quartet -- the European Union, Russia, the United States -- to work with the United Nations and quickly come forward with a proposal that will enable the Quartet to take up its mediation role and work jointly with countries in the region to advance the prospect of peace.
Israel must abandon the threats of annexation. The Palestinian leadership should re-engage with all members of the Quartet. Everyone must do their part, said Mladenov.
"I firmly believe that the time has come for all sides to do their part in the coming weeks and months in order to preserve the prospect of a negotiated two-state resolution to the conflict, in line with relevant UN resolutions, international law and bilateral agreements," he said. "These efforts must begin immediately, there is no time to lose. The fate of the Palestinian and Israeli people must not be determined by destructive unilateral actions that cement divisions and may put peace beyond our reach in our lifetime."