U.N. envoy Kofi Annan has asked governments with influence to "twist arms" to stop escalating violence in Syria, amid few signs that international pressure is having any measurable effect on the fighting.
Syria has intensified its onslaught against the opposition in recent days, ignoring an Annan-brokered ceasefire plan, mounting international condemnation and increasing economic pressure aimed at the government of President Bashar Assad.
On Tuesday, at least 10 people died as Syrian forces hit the eastern city of Deir el-Zour with mortar shells after an anti-government protest, and clashes broke out elsewhere in the country, activists said.
"It is totally unacceptable and it must stop, and that is why Annan has invited governments with influence to raise the bar to another level, to the highest level possible, and twist arms if necessary, to get the parties to implement the plan," his spokesman Ahmad Fawzi told reporters in Geneva.
He didn't specify the countries that might still have leverage with the Assad regime, but Russia, China and Iran are considered Syria's closest and strongest allies.
Fawzi said it was up to the government to take the first step to end the violence, which has now claimed over 13,000 lives since the conflict began in March 2011.
"The stronger party should send a strong signal in good faith and stop the violence, and the stronger party in this case is clearly the government of Syria," he said.
Annan, the former U.N. Secretary-General, has been working with little success to end the Syrian conflict since he was appointed in February.
Diplomats have discussed the possibility of holding a meeting of all the parties in a neutral venue, to attempt to restore the faltering truce and implement the peace plan brokered by Annan in April.
"We hope that this contact group meeting will take place soon, but a venue and a time and a list of participants has yet to come together," said Fawzi.
"The objective of creating this group is to give teeth to the plan, to convince the parties to implement the plan in its entirety," he added.
Britain's Foreign Secretary William Hague on Tuesday played down suggestions that Western powers were seeking a foreign military intervention, despite his warning on Monday that all options must remain open if diplomacy cannot halt the violence in Syria.
"Clearly, we are not looking for any foreign military intervention and we should not think about it in terms of another Libya," Hague said in Islamabad, during a two-day visit to Pakistan.
In recent days U.N. observers in Syria have documented heavy fighting in Rastan and Talbiseh, north of Homs, in which artillery, mortars and helicopters were used.
"They also reported that the Free Syrian Army had captured Syrian army soldiers," he added. "They reported a large number of civilians including women and children trapped inside Khaldiyeh in the city center and they are trying to mediate their evacuation."
Fawzi said the escalating violence mirrors the spike in fighting that occurred shortly before the ceasefire plan was agreed to, and the increase in sectarian tension is already spreading to neighboring Lebanon.
"The longer this violence continues the more dangerous it becomes not only for the country and the Syrian people but the region," he said. "It's dangerous and the red light is flashing."
The use of helicopters by government forces - a clear breach of the ceasefire - has been documented by the U.N. observers for the first time, said Fawzi.
"A ceasefire is a ceasefire, whether it's from the air or from the ground," he said.
David Stringer in London contributed to this report.