WASHINGTON - President Obama spoke by phone today with French President Francois Hollande about the situation in Syria, a day after a similar phone call with U.K. Prime Minister David Cameron.

Obama and Hollande discussed "possible responses by the international community" and "expressed their grave concern" about the reported chemical-weapons attack Wednesday outside Damascus, the White House announced.

On Saturday, Obama spoke via phone with Cameron, also discussing "possible responses," the White House announced.

France and the United Kingdom were America's two most prominent allies in 2011, when Obama sought to rally international support against the Libyan regime of Moammar Gadhafi. French, British, and U.S. forces launched airstrikes against Libya in March 2011, as a civil war raged there.

The White House sounded more certain this morning that the regime of Syrian President Bashar al-Assad was responsible for an attack that reportedly sent thousands to hospitals and killed hundreds. There is "very little doubt" that Assad's regime launched the reported attack, a senior administration official said.

Three hospitals supported by Doctors Without Borders reported witnessing symptoms consistent with a nerve-gas attack, the organization announced on Saturday.

Syria has blamed the attack on rebels, and after reports Sunday morning that Syria will allow U.N. inspectors to examine the site of the attack, the White House suggested that move may be too late, after "the regime's persistent shelling" has "significantly corrupted" evidence in the area of the reported attack.

"If the Syrian government had nothing to hide and wanted to prove to the world that it had not used chemical weapons in this incident, it would have ceased its attacks on the area and granted immediate access to the UN - five days ago. At this juncture, the belated decision by the regime to grant access to the UN team is too late to be credible," a senior administration official said.

Meanwhile, two top advisers were seen at the White House today, after the president convened a high-level meeting with his national-security team on Saturday - a session that included Vice President Joe Biden, Secretary of State John Kerry, CIA Director John Brennan, Director of National Intelligence James Clapper, Joint Chiefs of Staff Chairman Gen. Martin Dempsey, National Security Adviser Susan Rice, and others.

The president was presented with a range of options at that meeting, and Defense Secretary Chuck Hagel told reporters on Friday that the U.S. would prepare military options, including the repositioning of U.S. forces.

Today, Clapper and White House Chief of Staff Denis McDonough were seen entering the West Wing through the White House parking lot, although the White House did not announce any meetings.

As recently as Thursday, Obama has voiced caution about about military action, particularly without a U.N. mandate and international support.

Citing the war in Afghanistan that the U.S. is already fighting, and the potential for costly intervention that would breed more resentment of the U.S. in the Middle East, Obama suggested in a CNN interview that military action could be unwise without "clear evidence that can be presented" and "the coalition to make it work."