After nearly a year of failed international bids to halt Syria´s bloodshed, the U.N.´s top envoy to the country pushed to send hundreds of unarmed monitors into a combat zone to police a cease- fire that doesn´t exist.

Kofi Annan, the joint United Nations-Arab League envoy, told the U.N. Security Council on Tuesday that the monitors can reduce violence on the ground.

"Observers not only see what is going on, but their presence has the potential to change the political dynamics," Mr. Annan told the Security Council, according to his prepared remarks.

The council agreed, in a Saturday resolution, to send 300 monitors to Syria, responding to the failure of a cease-fire brokered by Mr. Annan to take hold two weeks ago. The resolution appeared to call for U.N. Secretary-General Ban Ki-moon to send the observers in once he had determined the situation was safe enough.

But a Security Council diplomat said Tuesday the body doesn´t expect an explicit safety briefing from Mr. Ban—but rather, that he could instead report if Syria became too dangerous for deployment, indicating the monitors are preparing to go in.

The proposition of sending in unarmed observers is unprecedented for the U.N., which is balancing monitors´ safety against broad international disbelief over reports that Syria´s government is carrying out reprisal killings. The risky calculationunderscores how few options are left to Washington and its European and Arab allies in their bid to end Syria´s bloodshed.

"When is enough enough?" asked Ahmed Fawzi, Mr. Annan´s spokesman. "That question is on everyone´s minds. This is not an open- ended process."

The U.N.´s Peacekeeping Department told the Security Council on Tuesday it would take at least a month to deploy the first 100 of the authorized 300 monitors, a Security Council diplomat said. It could take even longer if their Syrian visas are delayed. There are signs this is already happening, the diplomat said.

The U.N. observer mission was originally cast as a force deployed in part to monitor a cessation of hostilities that was to have taken effect on April 12. But the mission´s nature has quickly shifted. U.N. officials now say the team, which is expected to swell from about a dozen currently on the ground, will be deployed not to police a cease-fire but to help bring one about.

The Security Council diplomat said Mr. Annan´s six-point plan calls for monitors to deter violence, not just monitor a cease-fire, so the current proposal doesn´t change the mission´s mandate.

Mr. Annan, in his statements, cited concern over reports that government troops had been active in areas before and after observers´ visits. "Government troops entered Hama yesterday after observers departed, firing automatic weapons and killing a significant number of people," he said, citing reports that, if confirmed, would be "totally unacceptable and reprehensible."

Some Western officials and analysts said they fear the U.N. initiative will spin out for months, allowing Mr. Assad to buy time while continuing to crack down on opponents, a dynamic that Syrian opposition members have complained has already been occurring for months. During that time, the opposition has become both more militarized and more fragmented.

"They are engaged in the most extraordinary U.N. mission I´ve known, which is an unarmed observer mission to observe a peace that doesn´t exist," said Salman Shaikh, director of think tank the Brookings Doha Center, who has held posts at the U.N. "They are exposing themselves to risks and dangers that the members of the council and the secretary-general know, but nevertheless—in the absence of anything else—they persist."

If the Annan plan fails, Washington and its allies see few palatable options left to get Mr. Assad out, as the U.S. and European and Arab allies have sought, diplomats say.

"Nobody is really happy with the situation. No one really wants to send them in. But what are the choices?" the Security Council diplomat asked.

"What next: That´s the $60 million question," said another senior U.N. official. "There is no other plan on the table. What´s next is a protracted civil war."

Attacks by Syrian security forces on the city of Hama on Monday, which appeared to target opposition areas and activists who met with U.N. peace monitors a day earlier, have sharpened questions about the monitors´ mission.

Protesters and activists in Syria broadly compare the experience of the U.N. observers to those from an Arab League mission that failed earlier this year amid attacks that would set the stage for the conflict´s worst violence yet.

"This time, again, people feel like the monitors have brought death with them," said a resident of Arbaeen, the Hama neighborhood that several people there say came under attack on Monday. "The solutions they are presenting bring with them just more death."

On Tuesday, activists said the military continued to shell opposition strongholds throughout the country. The Damascus suburb of Douma was hit by artillery and machine-gun attacks, activists said, while fighting between security forces and opposition fighters roiled several other suburbs.

U.N. observers trying to visit the city of Homs on Tuesday were unable to enter certain districts because of fighting between rebel fighters and regime security forces, according to the opposition Avaaz group.

Syria´s government and an opposition group both reported an explosion in Damascus around midday Tuesday, which the government said was an explosive device placed under a car by a terrorist group. In the Damascus countryside, a retired lieutenant colonel and his brother were assassinated when an armed terrorist group opened fire on them, state agency Sana reported.

Mr. Annan´s spokesman, citing satellite imagery, said the Syrian regime hasn´t withdrawn all of its heavy weapons from populated areas as required by Mr. Annan´s cease-fire deal.

"They are claiming that this has happened. Satellite imagery, however, and credible reports show that this has not fully happened," Mr. Fawzi said in separate comments in Geneva. "This is unacceptable."

Mr. Assad´s international opponents at the U.N. are expected to continue to push for diplomatic pressure on Mr. Assad´s regime. But the individual sanctions route appears to have been exhausted, as the U.K. conceded on Monday.

Outside the U.N., the so-called Friends of the Syrian People coalition could step up its direct material support for the opposition, by establishing humanitarian corridors or providing military support to the opposition. At the group´s meeting in Istanbul earlier this month, the group took tentative steps in that direction. The U.S. pledged funds for humanitarian aid and said for the first time that it would send communications equipment to the rebels. U.S. officials also acknowledging that other states could be working to send lethal aid.

As recently as last month, it appeared Washington and its allies were banging war drums for a more robust intervention in Syria. But since then, the hawkish rhetoric has cooled some. Turkey, whose President Abdullah Gul warned Mr. Assad earlier this month that the Annan plan was his "last chance," after Syrian forces fired on a refugee camp inside Turkish territory, seems less intent on stoking conflict now.

"The reason why Turkey has become more cautious is directly linked to the cautiousness of the international community. Turkey was previously much more aggressive and ambitious but it remains isolated in the international community since major players chose to back the Annan plan," said Sinan Ulgen, a former Turkish diplomat now at the Carnegie Endowment for International Peace.

"Turkey is now becoming more realistic about its policy towards Syria; it´s becoming more conscious of the limitations of the tools at its disposal," he said.

—Joe Parkinson and Nour Malas contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/25/12)