"The SDF (Syrian Democratic Forces) have begun operations to clear the northern countryside, so this is putting pressure on Raqa," Baghdad-based US military spokesman Colonel Steve Warren said.
The US military will conduct air strikes in support of the "several thousand" SDF forces, some of whom have been trained and equipped by the United States.
Warren said the operation began earlier on Tuesday, and SDF forces had met little IS resistance across the sparsely populated area.
Approximately 3,000 to 5,000 IS fighters are in Raqa, Warren said, noting it was not clear when an assault on the key city itself might eventually come.
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A separate US official, speaking on condition of anonymity, said SDF troops were taking territory on their way to Raqa, but "they are not attacking Raqa" itself.
According to the Syrian Observatory for Human Rights monitoring group, coalition warplanes on Tuesday carried out dozens of strikes north of Raqa city.
If Raqa falls, "it's the beginning of the end of their caliphate," Warren predicted.
Russian Foreign Minister Sergei Lavrov on Tuesday said Moscow was ready to coordinate with both the United States and the SDF in the offensive for Raqa.
The SDF has a total of about 25,000 Kurdish fighters and about 5,000 Arab fighters.