A tornado that struck the Chicago area on Wednesday canceled more than 300 flights at the city’s two airports.

The twister touched ground near O’Hare International Airport on Wednesday afternoon, prompting the cancellation of 330 flights and delaying another 842, according to the FlightAware tracking platform. Some passengers sought shelter in a tunnel that connects concourses B and C at Terminal 1.

“This tornado has been touching the ground intermittently so far and is moving east,” the National Weather Service said on Twitter.

There were no immediate reports of injuries.

Sirens rang across Chicago, which had a tornado watch in place for 45 minutes starting at 7 p.m. local time, the Office of Emergency Management & Communications said in its Twitter account. Another 34 flights were canceled at the smaller Midway International Airport, and 340 were delayed, data from FlightAware showed.

More than 8,000 customers were affected by power failures in the Chicago area, utility ComEd said on its website.