The agency said it had requested further information from both parties to be submitted by March 5, after which it will assess whether their tie-up infringes Singapore's competition laws.

"CCS (Competition Commission of Singapore) is unable to conclusively determine that competition issues will not arise," it said in a statement.

ComfortDelGro said in a statement that "both parties remain committed to this partnership".

ComfortDelGro said in December it would buy a 51 percent stake in a unit of Uber that runs a fleet of private hire vehicles, as the companies seek to bridge the gap with dominant ride-hailing firm Grab.

(Reporting by John Geddie; editing by Jason Neely)