Dozens of M1's mobile subscribers were not able to connect to their 3G mobile network on Wednesday morning after a network upgrade caused "intermittent connectivity" in the north-western parts of the island.

In a statement, the telco said the outage occurred at 6.15am, shortly after a scheduled network upgrade to its radio network controller was completed.

"This affected some mobile customers in portions of Ang Mo Kio, Yishun, and Woodlands," said the telco's assistant general manager for corporate communications, Chua Hian Hou. Engineers did restoration works and full service was resumed by 9.15am, he added.

The Straits Times understands that this service outage is not linked to last week's SingTel fire which disrupted fibre broadband Internet, pay-TV, fixed and mobile voice services, as well as essential services such as banking, for hours. Earlier this month, M1 was fined a record $1.5 million for a service outage in January which lasted up to 71 hours, affecting some 250,000 M1 users - the worst mobile network failure Singapore has experienced to date.