BRUSSELS — Many flights between Europe and North America were either delayed or canceled Saturday due to the spreading cloud of volcanic ash stretching across much of the northern Atlantic, the European flight control agency said.

Flights had to be rerouted north over Greenland or south over Spain to avoid the 1,200-mile cloud from Iceland to northern Spain, Eurocontrol said. This increased flying times by about an hour in both directions.

"We assume that basically most of the trans-Atlantic flights will have to be rerouted on Saturday," Eurocontrol spokeswoman Kyla Evans said. "We expect substantial delays because of that."

About 600 airliners make the oceanic crossing every day. Around 40 percent will be rerouted southward.

The plume of ash also forced the closure of 15 airports in northern Spain on Saturday and is expected to expand into southern France during the day, carried along by Atlantic winds.

Just over 100 flights were canceled at mainland Portugal's three international airports on Saturday because of the ash cloud, the national airport authority ANA said.

Aer Lingus canceled flights from the United States to Dublin, citing the exceptionally circuitous routes to get around the cloud.