- The Syrian government killed 10 with a mortar attack on an eastern city, in defiance of a supposed ceasefire.
- U.N. envoy Kofi Annan called for strong governments to take action and "twist arms" to stem the tide of violence.
- Angry crowds opened fire on U.N. observers in a rebel-held town.
- An aid envoy announced an increase in the number of Syrian refugees fleeing to northern Iraq.
Clinton warned that helicopters from Russia could "escalate the conflict quite dramatically", although Russia has maintained that it would not condone the use of outside force to interevene in the conflict.
Clinton called for Moscow to help push forward a political transition plan.
Mortar attack kills 10
Hopes for a ceasefire took a major blow when an anti-government protest was attacked with mortars by the Syrian government.
The attack on the eastern city of Deir el-Zour killed at least 10 people and was the latest act of defiance of the internationally brokered ceasefire, which was supposed to go into effect on 12 April but which has been ignored by both sides.
Annan calls on governments to twist arms
Annan, the former U.N. secretary-general, called on governments to take greater action to slow the escalation of violence in the 15-month conflict.
His spokesman Ahmad Fawzi said after the Deir el-Zour attack: "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."
He added that it was up to the government to take the first step toward halting the violence, with the death toll now topping 13,000.
"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 added.
U.N. observers under attack
Violence also broke out in the embattled rebel-held town of Haffa as U.N. observers were attacked by crowds firing guns and throwing rocks and metal bars.
Concerns have been raised that Assad's forces are planning a massacre in Haffa, while the UN. .is concerned that civilians are trapped.
The Britain-based Syrian Observatory for Human Rights told Associated Press that residents who attacked the U.N. vehicles were mostly regime loyalists.
Refugee numbers continue to rise
The ever-escalating conflict, which appears set to lead to all-out war, has caused floods of Syrian refugees to flee the country.
According to the International Organisation for Migration, more than 5,000 refugees have crossed into northern Iraq and the numbers are growing.
The refugees, mainly ethnic Kurdish Sunni Muslims, are living in camps, with several hundred taking shelter in mosques, after paying around $300 to be smuggled out of the country.
Close to 100,000 refugees have fled to Turkey, Lebanon and Jordan.