The US is working with the Kurdish-led rebel group Syrian Democratic Forces (SDF) to try to defeat IS.
In November, the coalition said it had begun an operation to capture Raqqa, which has been held by the jihadist group since January 2014.
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The US announced, in December, that it was sending 200 more military personnel for the operation.
A Kurdish official told Reuters news agency this week that the aim of the campaign was to seal off all roads to the city, including links with Deir al-Zour province, another IS stronghold.