For nearly two years, the Obama administration has avoided direct military assistance to Syrian rebels out of a strong belief that it would not necessarily tip the balance in their favor, would be difficult to justify under international law and would displease the majority of an American public weary of foreign wars.

The administration still feels little pressure from public opinion, which remains less than enthusiastic about intervention in Syria and is not deeply engaged in the crisis, according to senior administration officials and recent polls.

But a number of variables in the Syrian equation have changed in recent weeks, weakening the case of those inside the White House who have long argued that military involvement is a political non-starter and strengthening the position of those, including the State Department, who favor stepped-up action.

The Syrian government is alleged to have used chemical weapons, in defiance of President Obama’s “red line.” Warnings of chaos spreading across the Middle East have grown increasingly dire.

U.S. allies and partners supporting the Syrian rebels have campaigned for stronger leadership from Washington. A growing minority of congressional voices is loudly calling for more U.S. involvement. There is new confidence that the rebel Free Syrian Army can effectively use more weapons while keeping them out of the hands of extremists in the opposition ranks.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

Last week, senior administration officials said that Obama is moving toward direct aid and that a final decision is only weeks away.

Secretary of State John F. Kerry, who as a senator and chairman of the Foreign Relations Committee first argued a year ago for sending weapons to the rebels and establishing a safe zone inside Syria, traveled to Moscow on Monday to try to convince the Russians that U.S. reluctance is waning as fast as the situation on the ground is deteriorating and that Russia should cease aiding the Syrian regime and work toward a negotiated settlement.

Reported Israeli airstrikes in Syria over the weekend have complicated Kerry’s task. Russian President Vladimir Putin spoke by telephone Monday with Israeli Prime Minister Benjamin Netanyahu, and a Foreign Ministry spokesman in Moscow warned that “internationalization of the extremely dangerous and devastating internal conflict in Syria must not be allowed.”

Another complication came with an allegation by Carla Del Ponte, a leading member of a U.N. commission investigating human rights abuses in Syria, that the rebels may have used chemical agents, as the government of Syrian President Bashar al-Assad has charged.

“Of course it’s been complicated,” a senior State Department official said of Kerry’s scheduled meeting with Putin. “But all the more reason that it comes at an opportune time.” Several administration officials spoke only on the condition of anonymity to discuss internal deliberations.

The administration was quick to differ with Del Ponte’s remarks. “We are highly skeptical of suggestions that the opposition could have or did use chemical weapons,” White House spokesman Jay Carney said. “We find it highly likely that any chemical weapon use that has taken place in Syria was done by the Assad regime.”

The United States, Britain, France and Israel have said there is persuasive evidence that Assad’s forces used sarin, a nerve agent, against rebel fighters. But the countries have called for additional evidence to be collected by U.N. investigators, who have not yet set foot in Syria.

The longer it takes for the investigation to begin, the less likely it is that evidence will be found, said Ake Sellstrom, the Swedish scientist tapped to head the U.N. team. Sellstrom has informed top U.N. diplomats that he is in a race against time and that the key signatures of a chemical attack — traces of chemical agents captured in soil and in human blood, hair and tissues — will be increasingly difficult to obtain as each day passes.

Although officials have said that Obama may act even without confirmation of chemical weapons use, polls indicate that it would weigh heavily on public opinion. A late-April survey by the Pew Research Center found for the first time that more Americans favored than opposed the United States and its allies taking action against the Syrian government, 45 percent to 31 percent, if evidence shows that Assad used chemical weapons against opposition groups.

One influential Democrat, Senate Foreign Relations Committee Chairman Robert Menendez (N.J.), said Monday that Assad has “crossed a red line,” and he introduced legislation that would give Obama expanded authority to supply arms and military training to the rebels. “The U.S. must play a role in tipping the scales toward opposition groups and working to build a free Syria,” Menendez said.

As the administration has contemplated its options, international law and human rights blogs have been filled with debate over whether intervention, through arms supplies or airstrikes, would be legal.

Russia and China have refused to endorse a U.N. Security Council resolution authorizing military involvement. Among other options, recognition of the opposition as Syria’s legitimate government would allow the rebels to officially request aid but would set a precedent that might cause problems in other countries and other revolutions.

Some legal experts say intervention under an international humanitarian law doctrine that some countries have recognized but others — including the United States, Russia and China — have not would set another risky precedent.

Finally, Obama could take a page from former president Bill Clinton, who sent U.S. warplanes to the Balkans, and offer no firm legal justification beyond the need for action based on facts on the ground.

Obama would have to define for Congress the threat to U.S. interests that required intervention.

Anne Gearan, traveling with Kerry, and Colum Lynch at the United Nations contributed to this report.