"It is extremely important that a solution to the current crisis be found by Friday this week," said Nickolay Mladenov after briefing the UN Security Council.
"The dangers on the ground will escalate if we go through another cycle of Friday prayer without a resolution to this current crisis," he warned.
The Security Council met behind closed doors to discuss ways to defuse tensions at the Haram al-Sharif mosque compound, known to Jews as the Temple Mount.
The Palestinians have denounced the measures as a bid by Israel to assert control over the holy site and five Palestinians were killed during clashes at the weekend.
Egypt, France and Sweden requested the council meeting as US President Donald Trump's envoy Jason Greenblatt arrived in Israel for talks on easing tensions.
Mladenov said he urged council members to use their influence with Israel and the Palestinians to encourage them to de-escalate tensions and to preserve access for worshippers.
While the violence is taking place "over a couple of hundred square meters... They have the potential to have catastrophic costs well beyond the walls of the Old City, well beyond Israel and Palestine, well beyond the Middle East itself," he warned.
Israel's Ambassador Danny Danon told reporters that Prime Minister Benjamin Netanyahu was working to calm the situation, but asserted that "we will do whatever is necessary to maintain security."
Palestinian ambassador Riyad Mansour accused Israel of "putting obstacles in the path of worshippers" and said the council must demand that the metal detectors and cameras be removed "completely and without conditions."
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