* Fire shifts aggressively on Tuesday to breach city limits
* Authorities issue mandatory evacuation order for Fort McMurray
* Suncor says cutting output to allow employees to get to safety (Adds Suncor production impact, Shell and Nexen statements)
By Nia Williams
CALGARY, Alberta, May 3 (Reuters) - Alberta is racing to evacuate thousands of people as an uncontrolled wildfire burns near Fort McMurray, in the heart of Canada's oil sands region, forcing residents to flee north to safety on Tuesday.
Alberta appealed for help from other provinces and Ottawa to help fight the fire and airlift people from the city. Authorities issued a mandatory evacuation order for all of Fort McMurray, which affects the city's 80,000 residents.
The 2,650-hectare (6,540-acre) fire, which was discovered on May 1, shifted aggressively with the wind on Tuesday afternoon to breach city limits. The blaze closed off the main southern exit from the city, prompting residents to head north towards the oil sands camps.
"This is the biggest evacuation we have seen in the history of the province," Alberta Premier Rachel Notley said at a news conference, adding that there were no casualties in the fire.
"We need to find more camps, we have secured spaces for about 6,000 people, we know we need to find more and that work is underway," Notley said.
Alberta is much drier than normal for this time of year, strengthening prospects for a long and expensive wildfire season, in the wake of a mild winter with lower-than-average snowfall and a warm spring.
Suncor Energy, whose oil sands operations are closest to the city, said its main plant, 25 km (16 miles) north of Fort McMurray, was safe, but it was reducing crude production in the region to allow employees and families to get to safety.
Suncor said evacuees were welcome at its Firebag oil sands facility, while Canadian Natural Resources Ltd said it was working to ensure any affected CNRL workers and their families could use its camps.
Shell Canada also said it would open its oil sands camp to evacuees and was looking to use its airstrip to fly out non-essential staff and accommodate displaced residents.
A number of flights from Fort McMurray airport were cancelled and the airport advised passengers to check with their airlines for updates.
The blaze started southwest of Fort McMurray and spread rapidly to the outskirts of the city. Radio stations were forced off the air as staff left the downtown core.
"There's lots of smoke, it's quite bad and hanging over the city," one resident, Nick Sanders, told Reuters as he packed up to leave downtown. "Where there are trees in the distance you can see the fire."
Television footage and photographs on Twitter earlier showed flames and smoke billowing over the city and traffic heading north on the highway to safety, while CTV News reported a trailer park had been destroyed.
"My thoughts are with people affected by the fire in Fort McMurray tonight," Canadian Prime Minister Justin Trudeau tweeted. "Stay safe and remember to follow evacuation orders."
By late afternoon, the fire had blocked off one major route out of town, closing Highway 63 south of downtown.
"Tomorrow is expected to be a more intense burning day," said Bruce Mayer, assistant deputy minister of Alberta's Forestry Division.
Nine air tankers, more than a dozen helicopters and more than 100 firefighters were battling the fire, he added.
Authorities on Tuesday said the wildfire jumped the Athabasca river and breached Highway 63, the main artery south from the isolated city, located about 430 km (267 miles) northeast of Alberta's capital, Edmonton.
Authorities now expect a cold front to reach Fort McMurray by Wednesday afternoon, bringing increased winds that will make it harder to fight the fire.
Most oil sands facilities are to the north and east of the city. Representatives of Syncrude, CNOOC subsidiary Nexen Energy and pipeline company Enbridge all said their operations were unaffected.
The fire is the second major one in the oil sands region in less than a year. Last May, wildfires led to the evacuation of hundreds of workers from the region, and a 9 percent cut in Alberta's oil sands output.
Last year's evacuations led to shutdowns by Cenovus Energy and Canadian Natural Resources Ltd at some of their projects. (Additional reporting by Euan Rocha and Allison Martell in Toronto; Writing by Amran Abocar; Editing by Bernard Orr and Clarence Fernandez)