Kurdish-led Syrian forces, backed by U.S. air power and military advisers, captured a number of Islamic State positions amid heavy fighting Monday north of the IS stronghold of Raqqa.
But the Syrian Observatory for Human Rights says there has been "no real progress" so far in the offensive.
"We do not underestimate the hard work ahead," Pentagon spokesman Peter Cook said Monday, contending that Syrian Democratic Forces (SDF) have been making progress in the early days of the advance, supported by U.S.-led coalition airstrikes.
The Pentagon also tried to again downplay concerns about any apparent rush to move on Raqqa from the ground, despite some opposition from allies like Turkey, which had said any move on Islamic State's Syrian capital should wait until after operations to retake the terror group's last Iraqi stronghold of Mosul had wrapped up.
"This is about applying pressure on multiple fronts, and we have been doing that from the air for some time," Cook told reporters.
"This is something that we have built toward, something we have planned for," he added. "This is where the campaign plan was supposed to be at this moment in time and this is not a surprise to us. This is exactly how it was sketched out."
A commander of the SDF had announced the start of the offensive on Sunday to gain control of Raqqa, the northern Syrian city overrun by Islamic State extremists in 2014 and self-designated as the center of extremist rule.
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Britain, France provide air support
Britain and France said Sunday they would also provide air support for the offensive by the Kurdish-led SDF, which also includes Arab fighters.
"We're supporting Iraqi-led forces defeating Daesh in Mosul and the RAF [Britain's Royal Air Force] will support the Raqqa operation as it develops," British Defense Secretary Michael Fallon said Monday, using an alternate name for the armed extremist group. "Our strikes are making a difference in Mosul now, and our sophisticated surveillance effort is helping give the coalition a vital edge in Syria."
The U.S.-led coalition is relying on the Syrian Democratic Forces, which is comprised of mostly Kurdish fighters, to isolate Raqqa, but the question remains as to whom will fight for the city and hold it once IS has been pushed out.
The U.S. Joint Chiefs of Staff, Gen. Joseph Dunford, met in the Turkish capital Sunday with his Turkish counterparts, including Gen. Hulusi Akar, chief of the Turkish general staff, for talks on the new offensive.
Dunford said Sunday the U.S. and its coalition allies are still working "to find the right mix of forces" for taking and governing the city.
Dunford's discussions in Turkey appear aimed at easing Turkish concerns about the makeup and objectives of the SDF, which Washington considers the most potent anti-IS fighting coalition in territory near the Turkish border.
For its part, the Ankara government views the main SDF component — the Syrian Kurdish fighting force known as the People's Protection Units — as a terrorist organization with links to another Kurdish force fighting government forces in southeast Turkey for regional autonomy.
Turkish officials went so far last week as to suggest that Turkish-backed forces take the lead in the push to free Raqqa rather than the Kurdish-led SDF.
A Pentagon statement late Sunday said the two generals agreed to "consult closely" on the coalition plan to seize and hold Raqqa, and to work collaboratively to disrupt IS operations in Syria and in Iraq.
U.S. Defense Secretary Ash Carter, speaking Sunday, said the international coalition will continue to "do what [it] can to enable local forces in both Iraq and Syria to deliver [Islamic State] the lasting defeat it deserves."
VOA National Security Correspondent Jeff Seldin and VOA Pentagon Correspondent Carla Babb contributed to this report.