Kofi Annan has warned the UN Security Council that the Syrian crisis will soon spiral out of control and called for consequences for the undermining of the peace plan.
Mr Annan briefed the closed-door session of the Security Council along with UN Secretary-General Ban Ki-moon.
International mediator Kofi Annan said hopes for consolidating the six-point peace plan for Syria were fading.
"The longer we wait, the darker the future looks for Syria," the international envoy also told the 15-member council."
Another diplomat quoted Mr Annan as saying the council must apply "united pressure" on Assad, the international envoy added.
Mr Annan again highlighted that his peace initiative could not be "open-ended".
The Security Council has passed two resolutions which approved the UN monitoring mission in Syria and condemned the violence there. But it is divided over how to increase pressure.
Russia, Syria's last major ally, and China have vetoed two council resolutions which only hinted at future sanctions. The United States and European nations want economic sanctions on Assad.
Earlier, UN monitors trying to get to the scene of a new massacre in Syria were shot at, UN leader Ban Ki-moon said, calling the latest atrocity "shocking and sickening".
Mr Ban told of the attack on the UN monitors in a speech to the UN General Assembly when he said that Syria's President Bashar al-Assad had "lost all legitimacy."
France to host July meeting
Meanwhile, France will host a meeting on 6 July of countries that back the departure of the Syrian president.
Foreign Ministry spokesman Bernard Valero confirmed Paris would host a third "Friends of Syria" meeting as world powers seek a way to stop the bloodshed in the uprising against President Assad.
About 50 nations, including the United States, Britain, Turkey, Saudi Arabia and Qatar, will take part.
However, Iran will not be be involved.
Read RTÉ Deputy Foreign Editor Anthony Murnane's analysis of the Syrian conflict.
