Syria´s government accepted a deadline to stop its attacks on the opposition by next week—Damascus´s highest-stakes acceptance yet of efforts aimed at ending the country´s conflict and one that drew guarded optimism from top diplomats after past failures to end the bloody crisis.

Syria´s government agreed to begin withdrawing its forces "immediately" and complete the pullback by April 10, according to Susan Rice, the U.S. ambassador to the United Nations. Ms. Rice, this month´s Security Council president, spoke after the council was briefed by Kofi Annan, a joint U.N.-Arab League envoy to Syria. Mr. Annan´s diplomatic team is asking opposition forces to withdraw from towns and cities 48 hours after the government completes its pullback, Ms. Rice added.

The deadline appears to make April 10 a new fulcrum in the crisis. While it isn´t clear whether Mr. Annan has laid out penalties for noncompliance, any failure by Damascus to follow through by then would come as the international community is poised, as it hasn´t before been, to open its conduits for aid and money to the opposition.

On Sunday, participants at an 83-nation meeting in Istanbul pledged humanitarian aid and salaries to opposition fighters. The flow of cash to rebel fighters could begin to shift the balance against President Bashar al-Assad by providing money that fighters could conceivably use to buy arms from regional black-market operations.

Syria´s crisis, which started more than a year ago with peaceful demonstrations against President Assad´s regime, has escalated into an armed conflict that the U.N. says has killed more than 9,000 people. Past efforts to quell the violence have come to naught, as Damascus´s pledges have dissapated or fallen short with the opposition, and Russia and China have blocked a U.N. effort to push Mr. Assad aside.

Syria´s government said last week it would accept Mr. Annan´s six- point cease-fire plan, which also calls on the government to allow unhindered humanitarian aid. Unlike an earlier Arab League version, the Annan plan doesn´t call for Mr. Assad to step aside, which Damascus has welcomed as a victory against foreign plot to bring down the president.

Until Monday, no deadlines had been made public. Bashar Ja´affri, the Syrian ambassador to the U.N., said the April 10 deadline was reached through a "common accord" with the team of Mr. Annan. "We are committed to implementing the six-point plan," Mr. Ja´affri said. A Western diplomat said Mr. Annan had sought an April 7 deadline but settled for the later date.

Ms. Rice expressed skepticism that Mr. Assad´s government would follow through, a sentiment she said was based on past broken promises by the regime.

"The proof is in the actions, not in the words," she said. "Past experience would lead us to be skeptical and to worry that, over the next several days, that rather than the diminution of the violence we might yet again see an escalation of the violence. We certainly hope that is not so."

About 80 people died in fighting across Syria on Monday, according to the Local Coordination Committees, an opposition group. Fatal clashes and shelling were reported in Homs and Idlib province, according to the Syrian Observatory for Human Rights, a U.K.-based opposition group.

A blast hit central Damascus, injuring at least four people when it exploded near a police station Monday afternoon, Syrian state television reported. Another bomb hidden in a small kiosk exploded in Aleppo, killing one civilian, the Observatory said. Late Monday, the group added that rebels had overtaken the national hospital in Homs´s Jouret al-Shayah district, finding nearly 70 refrigerated bodies of activists killed in earlier fighting. Activists and human-rights groups have accused government forces of using state-run hospitals to torture and interrogate rebels.

As Syrian forces have routed rebels from several towns and cities over the past few weeks, international resolve has grown to support the rebels with both lethal and nonlethal means.

On Sunday in Istanbul, at a meeting nations billing itself as the Friends of the Syrian People, Turkey joined the United Arab Emirates, Qatar and Saudi Arabia in openly calling for the rebels to be armed.

Officials at the meeting also said at least three Gulf nations would contribute to a fund to pay salaries for defected soldiers. That assistance would amount to $30 million to $40 million a month, according to a senior Syrian opposition figure.

Asked Monday whether the U.S. was comfortable with its allies funding an uprising and essentially paying those who fight the Assad regime, State Department spokeswoman Victoria Nuland said: "We have not discouraged this initiative, and we are coordinating closely with them."

Antiregime activists have grown impatient with the lack of momentum to arm the opposition as the government continues its military operations in towns, villages and cities across Syria. On Monday, some rebel fighters said they took heart from the opposition Syrian National Council´s pledge on Sunday to begin to pay salaries to defectors, a move that marks the first formal commitment of funds to Syria´s rebels.

The opposition´s outgunned fighters rely mostly on defectors and weapons they draw from the military, as well some smuggled from neighboring countries. In recent weeks, they have complained of running out of ammunition.

"A regular flow of payments will make a very big difference," said one Syrian activist who has helped to smuggle weapons and supplies into northern Syria through Turkey.

Other antiregime activists said the money pledged at the Istanbul conference isn´t enough to make an impact. They add that U.S. and Gulf states need to send arms caches to the rebels, rather than rely on the shoddy weapons available in small quantities in the black market.

"It has become an armed struggle. So we´re asking for military support, and that means arms. We need rebels to be resupplied with arms and that doesn´t mean buying an AK-47 for $2,000 from the black market in Lebanon, because those AKs don´t work well anyway," said Hamdi Rifai, a New York-based opposition activist and the chairman of the United Syrian Americans.

Ms. Rice said Security Council members expressed a willingness to consider the Annan plan for a monitoring mission if violence does, in fact, cease. "Some members of the Security Council expressed concern that the government of Syria not use the next days to intensify the violence and expressed some skepticism about the bona fides of the government in this regard," Ms Rice told reporters.

It also remained unclear whether the rebels would take advantage of the 48-hour window following a Syrian government withdrawal to regain lost territory, jeopardizing the deal.

Ms. Rice quoted Mr. Annan as saying that a U.N. peacekeeping team will join his staff in Syria this week to prepare for a monitoring mission that would police a cease-fire. A Western diplomat said last week that the team would consist of about 250 unarmed U.N. observers, pulled from neighboring peacekeeping missions. Mr. Annan said a peacekeeping team joined by some of his staff will travel again to Syria this week to continue preparations for a potential monitoring and supervisory mission.

—Nour Malas in Istanbul contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/03/12)