Five world powers on the United Nations Security Council, including the United States and Russia, will meet later Wednesday in New York to talk about the crisis in Syria - the launch of a high-stakes diplomatic push to avert an American military strike.
Envoys from France, Britain and China will also be at the meeting, at United Nations headquarters. Each country has veto power in the Security Council, so consensus is critical if any resolution to the crisis is to come from the U.N.
The meeting comes a day after President Barack Obama addressed the country on Syria, announcing that he would put off a military strike and work with Russia, China and American allies to get Syria to give up its chemical weapons.
Russia reshaped the Syria crisis by proposing two days ago that Syria could try to avoid an American attack by handing its chemical weapons over to international control.
But there have already been signs that Russia will complicate such a process. On Tuesday, Russia blocked a resolution crafted by the United States, France and Britain that would have called on Syria to turn over the weapons and threatened U.N. military enforcement. Russia wants a renouncement of military force.
Secretary of State John Kerry is to meet Thursday in Geneva with his Russian counterpart, Sergei Lavrov.
In his address, Obama said that the Syrian government's use of chemicals, in an attack on rebels and civilians last month outside Damascus, presented a danger to American security and violated the world's conscience.
"What kind of world will we live in if the United States of America sees a dictator brazenly violate international law with poison gas and we choose to look the other way?" he asked.
In Congress, senators worked on a resolution Wednesday that would authorize a military strike if diplomacy fails, but it was not clear when it might come up for a vote. Several senators privately expressed skepticism that a vote would happen at all.
A previous resolution authorizing force passed the Senate Foreign Relations Committee last week, but its prospects in the full Congress were murky.
Sen. Harry Reid of Nevada, the Democratic majority leader, said on the Senate floor that he had discussed Syria with Sen. Mitch McConnell of Kentucky, the Republican leader, and that they were working on a way forward.
Sen. Jerry Moran, R-Kan., was among the unpersuaded.
"I remain strongly opposed to the use of military force in Syria," he said in a statement Wednesday. "It is becoming increasingly clear that the president does not have a coherent strategy for intervention."
Democrats in the House emerged from a classified briefing on Syria, and some said they were encouraged.
"I think what the Russians have proposed may turn out to be the best thing to come out of Russia since vodka," said Rep. Brad Sherman, a Democrat from California.
Two reports Wednesday underlined the severity of the crisis in Syria, which is more than two years into a civil war.
United Nations war crimes investigators, in a report covering May through July, said that Syrian government forces had massacred civilians and bombed hospitals, and that the entire country was a battlefield.
Opposition forces, including Islamist fighters linked to al Qaeda, have also perpetrated war crimes, including executions, hostage-taking and shelling of civilian neighborhoods, the investigators said.
"The perpetrators of these violations and crimes, on all sides, act in defiance of international law," said the report, by the U.N. commission of inquiry. "They do not fear accountability. Referral to justice is imperative."
The International Committee of the Red Cross called on the United States and Russia to take time during their talks on chemical weapons to address the obstacles to delivering aid in Syria.
Both sides in the Syrian civil war are preventing medical help from getting to the sick and hurt, said Peter Maurer, the committee president. He told reporters in Geneva: "We need political and diplomatic support for independent humanitarian action."
Israeli Prime Minister Benjamin Netanyahu said that Syria must be stripped of chemical weapons and that "the world must make sure that whoever uses weapons of mass destruction pays a price for it."
"The message that is received in Syria will be received loudly in Iran," he said. Israel is deeply worried about the nuclear ambitions of Iran, and Netanyahu has said that Iran will only curtail its nuclear activity if it faces a military deterrent.