* Pipe on TransCanada mainline exploded late Saturday
* Residents return home as fire burns out
* TransCanada: service to customers not seen impacted
* Other pipes in area not believed damaged (Updates with safety agency comment, one pipe restarted)
VANCOUVER, Feb 20 (Reuters) - A natural gas pipeline ruptured in northern Ontario, igniting a fireball that sent flames hundreds of feet into the air, but service was slowly being restored, officials said on Sunday.
A section of the TransCanada Corp (TRP.TO) mainline exploded near Beardmore, Ontario late on Saturday, forcing "a few hundred" people to briefly evacuate their homes, the Ontario Provincial Police said.
No injuries were reported, and the fire burned itself out after several hours, Sgt. Greg Moore said.
TransCanada said the rupture involved pipe No. 2 of its three-pipe mainline system linking western and eastern Canada, and pipes No. 1 and No. 3 do not appear to have been damaged in the incident.
Pipe No. 1 was reopened on Sunday and No. 3 was expected to restart operations on Monday, according to Larry Gales, who is in charge of Transportation Safety Board's investigation of the incident.
"Service to customers is not expected to be impacted," said TransCanada spokesman James Millar.
The explosion lit up the night sky over the community about 170 km (110 miles) northeast of Thunder Bay, Ontario, with media in the area reporting the fireball could be seen by passing aircraft.
"Witnesses said the flames could be seen hundreds of feet in the air," Moore said.
The rupture's location made it difficult for investigators to reach the site, so it was impossible to predict how long it would take to repair the damage and determine the cause, Gales said. (Reporting Allan Dowd, Editing by Dale Hudson and Diane Craft)