National Security Advisory Susan Rice reaffirmed US commitment to supporting Syria's moderate opposition at a meeting in Washington with a delegation from the Syrian National Coalition, the White House said Saturday.

The Friday meeting came as the United States and coalition partners broaden air strikes against the Islamic State group in Syria, with a defense official telling AFP that "near continuous" combat sorties were being conducted there, as in Iraq.

Rice emphasized that US-led air strikes in Syria "are targeted at ISIL," and discussed ways to "support the moderate opposition to advance our mutual goals of countering ISIL and strengthening the prospects for a political transition," the statement said, using an alternate acronym for the IS group.

In addition to the air campaign, Washington is planning to train and arm 5,000 Syrian rebels to fight the IS organization.

Top US military officer General Martin Dempsey, however, has said between 12,000 and 15,000 men would be required to recapture "lost territory" in eastern Syria.

Washington has been keen not to let President Bashar al-Assad's forces take the upper hand by exploiting the anti-IS air campaign.

Rice "reiterated that President Assad had lost all legitimacy" as she met with the delegation, led by SOC President Hadi al-Bahra, and emphasized the US commitment to a negotiated settlement "that ensures a genuine and broadly inclusive political transition."

The US and Arab allies launched air strikes against IS in northern and eastern Syria on Tuesday, more than a month after Washington began its air war on the jihadists in Iraq.

The Syrian National Coalition is the main exiled opposition group seeking the overthrow of Assad, amid the more than three-year-old civil war.