WASHINGTON — The Obama administration, concluding that the troops of President Bashar Assad of Syria have used chemical weapons against rebel forces in his country’s civil war, has decided to begin supplying the rebels for the first time with small arms and ammunition, according to U.S. officials.

The officials held out the possibility that the assistance, coordinated by the CIA, could include anti-tank weapons, but they said that for now supplying the anti-aircraft weapons that rebel commanders have said they sorely need is not under consideration.

Supplying weapons to the rebels has been a long-sought goal of advocates of a more aggressive U.S. response to the Syrian civil war. A proposal made last year by David Petraeus, then the CIA director, and backed by the State Department and the Pentagon to supply weapons was rejected by the White House because of President Barack Obama’s reluctance to be drawn into another war in the Middle East.

But even with the decision to supply lethal aid, the Obama administration remains deeply divided about whether to take more forceful action to try to quell the fighting, which has killed more than 90,000 people in more than two years. Many in the U.S. government believe that the military balance has tilted so far against the rebels in recent months that U.S. shipments of arms to select groups may be too little too late.

Some senior State Department officials have been pushing for a more aggressive military response, including airstrikes to hit the primary landing strips in Syria that the Assad government uses to launch the chemical weapons attacks, ferry troops around the country and receive shipments of arms from Iran.

But White House officials remain wary, and on Thursday Benjamin Rhodes, one of Obama’s top foreign policy advisers, all but ruled out the imposition of a no-fly zone and indicated that no decision had been made on other military actions.

Obama declared in August that the use of chemical weapons by the Syrian government would cross a “red line” that would prompt a more resolute U.S. response. In an April letter to Congress, the administration said that intelligence agencies had “varying degrees of confidence” that Syrian government troops had used chemical weapons. But the conclusion of the latest intelligence review, according to officials, is more definitive.

White House officials said Thursday that the Assad government had used chemical weapons “on a small scale against the opposition multiple times in the last year.” The assessment came after U.S. and European government analysts examined physical evidence and other intelligence indicating that Syrian troops had used sarin nerve gas against the opposition.

The announcement said that U.S. intelligence officials now believed that 100 to 150 people had died from the attacks, but officials cautioned that the number could be higher.

That conclusion is based on evidence that includes intelligence on the Assad government’s plans for the use of chemical weapons, accounts of specific attacks, and descriptions of symptoms experienced by victims of the attacks. Rhodes said the new assessment had changed the president’s calculus.

But the president’s caution has frayed relations with important U.S. allies in the Middle East that have privately described the White House strategy as feckless. Saudi Arabia and Jordan recently cut the United States out of a new rebel training program, a decision that U.S. officials said came from the belief in Riyadh and Amman that the United States has only a tepid commitment to supporting rebel groups.

Moreover, the United Arab Emirates declined to host a meeting of allied defense officials to discuss Syria, concerned that in the absence of strong U.S. leadership the conference might degenerate into bickering and finger-pointing among various Gulf nations with different views on the best ways to support the rebellion.

Adding to those voices was former President Bill Clinton, who earlier this week endorsed a more robust U.S. intervention in Syria to help the rebels, aligning himself with hawks like Sen. John McCain, R-Ariz., who fault Obama for his reluctance to get entangled in the war.

Speaking Tuesday at a private session in New York with McCain, Clinton said, “Sometimes it’s best to get caught trying, as long as you don’t overcommit.”

A flurry of high-level meetings in Washington this week underscored the splits within the Obama administration about what actions to take in Syria to stop the fighting. The meetings were hastily arranged after Assad’s troops, joined by thousands of fighters from the militant group Hezbollah, claimed the strategic city of Qusair and raised fears in Washington that large parts of the rebellion could be on the verge of collapse.

After weeks of efforts to organize a conference at which the Assad government and the opposition were to negotiate a political transition, the administration is now slowing down that effort, fearful that if it were held now, Assad would be in too strong a position to make any concessions.

The conference has been pushed back repeatedly amid warnings that the main rebel leaders did not plan to attend. But now, an administration official said, the focus will switch from setting a date to fortifying the rebels before they sit across the table from the government.

The timing of the announcement Thursday on the use of chemical weapons, an official said, reflected both the completion of the intelligence assessment and the fact that Obama leaves Sunday for a meeting of the Group of Eight industrialized countries in Northern Ireland.

Formally designating the Assad government as a user of chemical weapons, this official said, will make it easier for Obama to rally support from Britain, France and other allies for further measures.

Until now, the U.S. support to Syrian rebels has been limited to food rations and medical kits, although the Obama administration has quietly encouraged Saudi Arabia, Qatar and Turkey to ship weapons into the country.

The limited assistance that the Obama administration is now promising is likely to be dwarfed by the help that U.S. officials said that Iran provides to Assad’s government. Many of the weapons and other military assistance that Iran has provided have been flown to Damascus from Iran through Iraqi airspace, the officials said.

There was a lull in Iranian flights after Secretary of State John Kerry pressed Prime Minister Nouri al-Maliki of Iraq during a March visit to Baghdad to order more inspections of Iranian flights that crossed Iraqi airspace. By early May, however, the Iranian flights had started up again.

The flights, and Hezbollah’s decision to enter the fight on the side of the Assad government, have provided such an important boost for the Assad government that some senior State Department officials believe that Assad’s gains cannot be reversed unless the United States takes steps to curtail the Iranian arms flow, disabling the airfields that the Syrian government uses to receive arms, transport troops around the country and carry out air attacks.

Rhodes said there was no reason to think that the resistance has access to chemical weapons.

“We believe that the Assad regime maintains control of these weapons,” he said.

According to a CIA report, which was described by a U.S. official who declined to be identified, the United States has acquired blood, urine and hair samples from two Syrian rebels — one dead and one wounded — who were involved in a firefight with Syrian government forces in mid-March northeast of Damascus. The samples showed that the rebels were exposed to sarin.

Syrian death toll — Civilians are bearing the brunt of the fighting in Syria, Navi Pillay, the U.N. high commissioner for human rights, said Thursday, with 92,901 killings documented there through the end of April, a number that may understate the magnitude of the violence that has devastated cities and villages across the country for 25 months. “The constant flow of killings continues at shockingly high levels,” Pillay said in a statement in Geneva, “with more than 5,000 killings documented every month since last July, including a total of just under 27,000 new killings since Dec. 1.” Pillay cautioned that the analysis was conservative, and that “the true number of those killed is potentially much higher.” The latest study builds on an earlier report that found about 60,000 documented deaths through Nov. 30, 2012. The report was not able to break down the deaths by combatant and noncombatant, or pro- and anti-government forces.

— New York Times News Service