"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 today, and SDF forces had met little IS resistance across the sparsely populated area.
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.
More From This Section
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.