International leaders meeting here Friday agreed on a unified plan for pressure they hope will stop Syrian President Bashar al-Assad’s bloody crackdown on civilian opponents and drive him from power, but they stopped well short of approving military assistance to the Syrian opposition.

The “Friends of Syria” gathering agreed to tighten sanctions against Assad and his government, and called on the United Nations to ready a peacekeeping force for deployment, along with massive international reconstruction assistance once the ongoing violence ceases and, presumably, Assad has stepped down.

A year into a popular uprising that has claimed some 6,000 Syrian lives, the lack of agreement on tougher measures reflected misgivings about further militarizing a conflict that some fear could ignite a wider war in the heart of the Middle East.

The session came as President Obama said in Washington that the United States and its allies would consider “every tool available’’ to halt the killing in Syria, some of the strongest language he has used during the crisis. For her part, Secretary of State Hillary Rodham Clinton told the gathering that “we cannot wait for this crisis to become an even greater catastrophe.”

But despite their calls for an “immediate” end to the government bombardment of the city of Homs and other opposition strongholds to allow humanitarian assistance to safely enter the country, the participants here voiced few illusions that their actions would produce immediate results.

The united denunciation of Assad masked disagreement in the closed-door conference on whether to arm opposition forces. Asked whether he favored supplying weapons to the Free Syrian Army composed primarily of military defectors, Saudi Arabian Foreign Minister Saud al-Faisal, said, “I think it’s an excellent idea.” He spoke to reporters as he entered a private meeting with Clinton.

But Tunisian Foreign Minister Rafik Abdessalem, the conference host, noted that the final conference communique included no mention of military aid or outside intervention. “It is essential to have a safe transfer of power,” Abdessalem said, making clear his own government’s opposition to lethal aid. “We don’t want it to slip into civil war.’’

Clinton used sharply undiplomatic language to denounce Russia for its support of the Assad regime and its veto, along with China, of a U.N. resolution this month calling for him to step down.

“It is just despicable,” Clinton said, “and I ask, whose side are they on? They are clearly not on the side of the Syrian people.”

The Obama administration has undergone its own metamorphosis in the wake of Russian intransigence and amid the still-rising civilian death toll in Syria a year into the popular uprising. As videos and other reports of the carnage have flooded the international media, and prominent journalists have died trying to report on it, Syria has also become an issue in the U.S. presidential campaign, with Republican leaders calling for support for arming those fighting against Assad.

This week, the administration moved from insisting that military aid to the opposition would be a mistake, to warning of “additional measures” if Assad did not yield. In London on Thursday, Clinton said armed elements of the opposition would be “increasingly capable” of launching offensives against government military forces.

The session here was attended by representatives from 70 countries, and some participants spoke of lessons learned during a year-old Arab Spring in which four longtime Arab leaders, including one in Tunisia, have been removed from power under vastly different circumstances. At the very least, another diplomat said, the conference was a major step toward positioning the international community for quick agreement on further steps if they become necessary.

“One cannot become discouraged or impatient,” Clinton said, warning that “there will be more killing before [Assad] finally goes.” She said the Syrian “Friends” should “stay focused . . . on what we agreed on” at the conference, including increased international pressure on Assad and those backing him.

Obama administration officials described the tougher language used by Clinton in recent days as useful to increase pressure on Assad. But they also made clear that steps toward arming the opposition were likely to become a reality they would not oppose if the Syrian leader does not yield.

Although no official arms shipments are going to the rebels — and no government is known to favor direct intervention by outside military forces — a number of countries in the region are known to be facilitating supplies purchased on commercial markets and planning for possible direct assistance.

Clinton held a private meeting with Burhan Ghalioun, president of the opposition Syrian National Council, which was given a seat at the Tunis table. U.S. officials, speaking on the condition of anonymity to describe the private sessions, remain concerned that the exile-led SNC has so far failed to gain the trust of minority groups who fear their position in a post-Assad Syria.

Reassurances to Syria’s military and civil service were also part of the conference agreement, as the administration and others seek to convince Assad’s supporters and institutions inside Syria that they are on the losing side and should quickly part company with him.

“They can make the guns fall silent,” Clinton said. “People around Assad are beginning to hedge their bets and are looking for ways out” and even Assad “can still make the choice to end the violence, save lives and spare his country from descending into ruin.”

To bring humanitarian and political efforts together, the United Nations announced on Friday the appointment of former U.N. secretary general Kofi Annan as a special envoy representing both the United Nations and the Arab League.

Annan will supervise the establishment of aid “hubs” in neighboring countries where stores of food, water and medical supplies are being assembled to enter Syria as soon as it is safe to do so. Among countries pledging additional resources to that effort, Clinton announced an initial U.S. contribution of $10 million.

Another blow to Assad came Friday when the Palestinian Islamist group Hamas for the first time turned publicly against its longtime ally. “I salute the heroic people of Syria who are striving for freedom, democracy and reform,” Hamas leader Ismail Haniyeh said.

The exiled political leadership of Hamas had been based in Damascus for over a decade but recently quit the Syrian capital.

Staff writers Alice Fordham in Beirut and Colum Lynch at the United Nations contributed to this report.