(CNN) -- Special envoy Kofi Annan told the U.N. Security Council on Tuesday that international observers are the last hope for success of a peace plan in Syria. Otherwise, he warned, Syria is sure to plunge into full-scale civil war.
"The implication of that is quite frightening," Annan told reporters after briefing the Security Council. "We cannot allow that to happen."
Annan plans to return to Damascus soon to seek adherence to a cease-fire. The killing, he said, must stop immediately.
"There has been some decrease in the military activities, but there are still serious violations in the cessation of violence that was agreed," Annan said.
Mark Lyall Grant, the British ambassador to the United Nations, said 60 observers were on the ground along with a staff of 37 civilians. He said the United Nations hoped to have 100 observers by the end of this week and 300 by the end of May.
He called Annan's briefing "fairly somber" and said it was clear that the peace plan brokered by Annan and agreed to by Syrian President Bashar al-Assad has not been fully implemented.
"We're some way from having the right environment to have a political dialogue," Lyall Grant said.
Susan Rice, the U.S. ambassador to the United Nations, said the United States stood behind its position that al-Assad has lost all legitimacy and must step down. If the violence persists, she said, the Obama administration is ready to look at other means to ratchet up pressure on Damascus, including a renewed discussion in the Security Council.
Annan's briefing came a day after U.N. Secretary-General Ban Ki-moon said the Syrian situation has become one of the "most serious and gravest concerns of the international community."
"More than 9,000 people have been killed during the last 14 months. This is totally unacceptable and an intolerable situation," Ban said.
But the killing hasn't stopped.
At least 17 people died Tuesday in Syria, including a soldier who was shot weeks ago in Aleppo while trying to defect and a young man shot by security forces in Hama, the opposition Local Coordination Committees of Syria said.
The LCC reported explosions in Damascus, Homs and Hama.
The Syrian ambassador to the United Nations again blamed the violence on armed perpetrators that he alleged were supported and financed by Turkey, Saudi Arabia, Qatar and other nations.
"We are still committed toward guaranteeing the maximum success to the mission of Kofi Annan," Bashar Jaafari said at a news conference Tuesday. "But the Syrian government cannot do all the job alone."
He said the international community was acting hypocritically by not acting against terrorists in Syria as they do against al Qaeda.
"This is when deeds follow the words," he said.
Rice said no one could say with certainty there were no foreign fighters in Syria. But, she countered, that was beside the point.
"This is substantially a diversion from the main point," she said. "The main point is that the government of Syria continues to kill its own people."
Amid the violence and pressure on al-Assad to stand down, the Syrian government touted a "wide turnout" for parliamentary elections Monday, when more than 7,000 candidates vied for 250 parliamentary seats.
But members of the opposition urged Syrians to boycott the elections, saying a vote for any of the candidates amounted to a vote for al-Assad, whose family has ruled the country for 42 years.
Spurred by the success of popular uprisings in Tunisia and Egypt, Syrians first took to the streets in March 2011 calling for reforms and increased political freedoms. But a brutal government crackdown against demonstrators saw the movement quickly devolve into a bloody uprising.
Both the Syrian regime and the rebel Free Syrian Army have agreed to a peace plan brokered by Annan, the U.N.-Arab League special envoy to Syria. A key element of the plan involves a cease-fire by all parties and the withdrawal of Syrian forces from populated areas.
But reports of attacks have only mounted since April 12 when the cease-fire was supposed to go into effect.
Annan's plan calls for the government to allow humanitarian aid groups access to the population, the release of detainees and initiation of political dialogue.
Many nations, including many of Syria's Arab neighbors, have condemned the ongoing violence, which has pitted a minority Alawite-dominated government against a predominantly Sunni uprising.
The LCC says more than 11,000 people have been killed in 14 months. CNN cannot independently verify reports of violence and deaths within Syria as the government has restricted access by most of the international media.
CNN's Joe Vaccarello and Moni Basu contributed to this report.