The U.N. Security Council called upon Syria on Thursday to “urgently and visibly” halt its attacks on opposition targets as special emissary Kofi Annan told the U.N. General Assembly that Syrian “military operations in civilian population centers have not stopped.”

The 15-nation council’s demand was aimed at bolstering Annan’s efforts to secure a cease-fire that would end the government’s year-long crackdown on dissent and prevent a descent into all-out civil war. It also appeared intended to press Syria to honor a four-day-old commitment to stop shelling residential areas and withdraw its heavy weapons and troops from urban areas by an April 10 deadline.

Annan, a former U.N. secretary general who is serving as the joint envoy for the United Nations and the Arab League, told the General Assembly that “clearly, the violence is still continuing.” He urged member states to use whatever influence they have on all sides in the conflict to stop the fighting and enter “Syrian-led” political talks that meet “the democratic aspirations of the Syrian people.”

“We must silence the tanks, helicopters, mortars, guns and stop all other forms of violence too,” Annan said in his address, delivered via videolink from Geneva. “I am impatient for action on commitments made. I hope both the government and the opposition understand what is at stake and seize this moment. Let us stop the killing and start serious political dialogue, for the well-being of the Syrian people.”

Members of the opposition speaking via Skype from Idlib province, which has seen heavy fighting in recent weeks, said that they had seen soldiers digging trenches and that none had withdrawn from urban areas. Activist groups reported a heavy military operation in Douma, the suburban area east of the capital that was briefly held by the rebels this year.

“We have received reports of 393 people killed since Syria accepted Annan’s plan,” said Ann Harrison of Amnesty International.

While Harrison stressed that it is difficult to rely on activists’ reports, she said that the rights group had received information about ongoing arrests — including of children — despite a stipulation in Annan’s plan that prisoners be released. Harrison added that there had also been reports of kidnappings and killings committed by opposition fighters — although far fewer in number than instances of brutality against civilians committed by the security forces.

The failure of President Bashar al-Assad to comply with Annan’s peace initiative has reignited the Obama administration’s hopes that Russia and China can be persuaded to support a more robust Security Council resolution, along the lines of the two they vetoed in recent months.

Once Annan determines that “we’re not getting any results . . . we would go back to the Security Council,” Secretary of State Hillary Rodham Clinton said Sunday in an interview with CBS News. “Now, what would Russia and China say?”

Annan, Clinton said, “has gone to Moscow, he’s gone to Beijing, he’s met with them. They support his plan. They have urged publicly that Assad follow the plan.”

“So, if we have to go back to the Security Council to get authority” for more assistance to the Syrian opposition, she said, “I think we’ll be in a stronger position than we would if [Annan] hadn’t had a chance to go and try to negotiate.”

The Security Council’s statement urges the Syrian government to “urgently and visibly implement its commitment” to cease the use of heavy weapons against opposition elements, to stop sending military forces into towns and to “begin pullback of military concentrations in and around population centers” by April 10.

It also calls on both the Syrian security forces and the armed opposition to “cease armed violence in all its forms” within 48 hours of the government completing its withdrawal from key towns.The statement throws the council’s full weight behind Annan’s six-point peace initiative and his plan to establish a small U.N. mission in Syria to monitor a possible cease-fire. “The Security Council underscores the importance of an effective and credible United Nations supervision mechanism in Syria to monitor the cessation of armed violence . . . by all parties,” the statement said.

Susan E. Rice, the U.S. ambassador to the United Nations, said that the “violence has intensified, and our understanding is that yesterday was one of the more violent days of late in many of the Syrian cities of concern.”

Rice, who is presiding this month over the council’s rotating presidency, said: “As I’ve said in the past, we will be looking at actions and deeds rather than their words.”

Annan said that he hopes a “full cessation of violence” can be achieved by 6 a.m. Damascus time Thursday and that he would need a “small and nimble” monitoring mission with “freedom of movement throughout the country” to assess what unfolds after that.

Correspondent Alice Fordham in Beirut and staff writer Karen DeYoung in Washington contributed to this report.

An earlier version of this story referred incorrectly to the language in the U.N. Security Council statement on Syria. The statement demanded that Syria “urgently and visibly” halt its attacks on opposition targets, not “immediately and verifiably” halt the attacks.