President Obama was given a detailed review of options Saturday for a potential U.S. response to last week’s reported chemical weapons attack in Syria, amid a buildup of U.S. military assets in the region.

No decisions were announced, and “the U.S. intelligence community continues to gather facts to ascertain what occurred,” according to a terse White House statement issued after a National Security Council meeting that included Vice President Biden and top defense, intelligence and diplomatic officials.

But ongoing assessments, it said, were “mindful of the dozens of contemporaneous witness accounts and records of the symptoms of those killed.” The administration has made clear its certainty that the Syrian government was responsible for Wednesday’s attack, which opposition activists have said killed hundreds, if not thousands, of people in a rebel stronghold east of Damascus.

Among the options at Obama’s disposal are cruise missiles launched from U.S. warships. A defense official, speaking on the condition of anonymity to discuss fleet deployments, said that a destroyer that had been scheduled to leave the Mediterranean was retained there to keep more resources in the area, bringing the total to four.

Other options range from sending sophisticated weapons to Syrian rebels to using U.S. air power to establish a no-fly zone over rebel-held areas. Cruise missiles, if they were deployed, would most likely be fired at Syrian military installations as a warning to the Syrian government of U.S. seriousness regarding chemical weapons use, rather than an attempt to immediately alter the balance in the ongoing civil war.

Among several calls that Secretary of State John F. Kerry made to counterparts in the Middle East last week regarding the attack was a rare call Thursday to Syrian Foreign Minister Walid al-Moualem, according to a senior State Department official who requested anonymity to discuss sensitive diplomacy.

In a Lebanese television interview Saturday, Syrian Information Minister Omran al-Zoubi warned that any U.S. attack would result in a “ball of fire that would burn not only Syria but the whole Middle East.”

President Bashar al-Assad’s government has denied using chemical weapons. Zoubi echoed suggestions by Russia, Assad’s main international backer, that rebels were responsible for the attack in the contested Ghouta area. “The rockets were fired from their positions and fell on civilians,” he said.

The Syrian news agency SANA said that Syrian soldiers in eastern Damascus were being treated for symptoms of “suffocation.”

Meanwhile, Angela Kane, head of disarmament issues for the United Nations, arrived in Damascus. She was dispatched by U.N. Secretary General Ban Ki-moon to press the government to allow international weapons inspectors already in the Syrian capital to investigate the attack.

Obama said last year that any use of chemical weapons by Assad, whose arsenal includes sarin nerve gas, would be a “red line” that would require a U.S. response. Some U.S. lawmakers criticized his caution in intervening to stop a two-year-old conflict which his own administration has said has cost the lives of more than 100,000 Syrians, largely at the hands of government forces.

In a CNN interview Friday, Obama cited the need to “think through strategically what’s going to be in our long-term national interests” and to consult with allies. He said that he also had to consider international law.

International law generally requires U.N. Security Council approval for any military action against a member state, but Russia has vetoed several council resolutions intended to bring pressure on Assad and is unlikely to approve any call for intervention. White House and State Department lawyers have been seeking options outside U.N. confines, and consulting with countries, including Britain, that might join in a Syria operation.

Obama discussed Syria in a call Saturday with British Prime Minister David Cameron, the White House said.

As the administration continues to assemble evidence about the chemical attack, the Britain-based Syrian Observatory for Human rights raised its estimated death toll from 136 to 322.

The international medical aid group Doctors Without Borders said that three hospitals it supports in Damascus reported seeing about 3,600 patients displaying “neurotoxicity symptoms” in less than three hours on the morning of the attack. Of those patients, it said, “355 reportedly died.”

Although its own personnel had not been able to visit the area, the group said in a statement that doctors at facilities there reported patients with symptoms including “convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress,” and said they were treated with atropine.

The organization, also known as Médecins Sans Frontières, “can neither scientifically confirm the cause of these symptoms nor establish who is responsible for the attack,” said Bart Janssens, director of operations. “However, the reported symptoms of the patients, in addition to the epidemiological pattern of events — characterized by the massive influx . . . in a short period of time, the origin of the patients, and the contamination of medical and first-aid workers — strongly dictate mass exposure to a neurotoxic agent.”

Ernesto Londoño in Washington and Anne Gearan contributed to this report.