The Washington Post’s Liz Sly and David Ignatius look back at the bloody Syrian civil war--thousands killed, a country in ruins and borders breached by a tide of refugees. What will the future hold for the Syrian people and the al-Assad regime, and how does the U.S. fit into that picture? (Jason Aldag/The Washington Post)

President Obama said Wednesday that the United States is still investigating whether chemical weapons were used in Syria and reiterated his pledge that their use by the government of President Bashar al-Assad would be a “game changer” for U.S. policy.

“We have to make sure that we know exactly what happened, what was the nature of the incident, what we can document, what we can prove,” Obama said at a news conference in Jerusalem with Israeli Prime Minister Benjamin Netanyahu.

“I won’t make an announcement today about next steps, because I think we have to gather the facts,” he said. In the past, Obama has called any use of such weapons by Assad a “red line.”

Obama’s statements came after the Syrian government and the opposition repeated their accusations that the other side used chemical weapons in an attack on a village near Aleppo on Tuesday. Both sides asked the United Nations to send a team to investigate the incident.

Although the facts have not been established, Obama said, he was “deeply skeptical of any claim that it was the opposition that used chemical weapons,” noting that only the regime has the capacity to carry out such an attack. The rebels said the chemicals were delivered on a Scud missile.

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

Obama also defended his administration’s reluctance to intervene directly to stop the bloodshed in Syria, telling an Israeli reporter, “It is incorrect for you to say that we have done nothing.”

The United States, he said, had helped mobilize the world to isolate Assad, provided hundreds of millions of dollars in humanitarian assistance, supported and recognized the Syrian opposition, and worked with other countries to “move towards a political transition.”

Obama’s comments, his first since the chemical-weapons reports were backed up by Robert Ford, the U.S. ambassador to Syria, who said at a House hearing Wednesday that “so far we have no evidence to substantiate the report that chemical weapons were used yesterday.”

“But I want to underline that we are looking very carefully at these reports,” Ford told lawmakers. “We are consulting with partners in the region and in the international community.”

The ambassador did not respond to repeated questions from the House Foreign Affairs Committee about what action the administration would take if Assad were proven to have used chemical weapons.

“I absolutely do not want to go into hypotheticals,” Ford said. “I do want to underline that we take these reports and these possibilities very seriously, and we are using all of our available means to determine exactly what has happened.”

The Britain-based Syrian Observatory for Human Rights said 26 people, including 16 Syrian soldiers, died in the Tuesday attack. But the observatory’s director, who uses the pseudonym Rami Abdulrahman, said he could confirm only “that there was a rocket attack but not that any chemicals were used.”

Photographs posted online by the official Syrian Arab News Agency showed alleged victims in hospital beds flanked by medical staff in surgical masks. State television featured an interview with an elderly man wearing a face mask and a white bandage on his forehead. “They fired a missile, and it exploded with something like a powder,” the man said.

In the same TV report, a doctor said the patients appeared to have been exposed to “phosphorus material or some poisonous material,” which he said had led to “heavy vomiting and difficulty in breathing, almost appearing like extreme suffocation cases.”

Syria is known to possess large arsenals of the deadly nerve agents sarin and VX. Symptoms of both include vomiting, and death is usually caused by asphyxiation as muscle paralysis impedes breathing. Both can contaminate through inhalation or skin contact, even with clothing worn by victims.

Intelligence reports in December that Syrian military units were moving toward possible activation of chemical arms led to a direct warning from Obama and subsequent administration assurances that the weapons were secure.

U.N. Secretary General Ban Ki-moon’s office said he was studying a written request from the Syrian government, backed by Russia, for an investigation into its claim of chemical-weapons use by the rebels.

France and Britain countered with an effort to get as many Security Council members as possible to sign a letter asking Ban to open a wider investigation into the use of chemical weapons in Syria. No evidence that such weapons were used was presented.

In addition to concerns about any use of chemical weapons against the Syrian people, Obama said the administration shared “Israel’s grave concern about the transfer of chemical or other weapons systems to terrorists, such as Hezbollah, that might be used against Israel.”

Babak Dehghanpisheh and Ahmed Ramadan in Beirut and Colum Lynch in New York contributed to this report.