A collision between two private buses caused a three-hour jam on the Kranji Expressway (KJE) early Friday morning.

Witnesses say the jam started before 6am, and traffic is still not back to normal at 9.30am. Preliminary checks with the Traffic Police and Land Transport Authority reveal that the entire expressway was closed between 6am and 8am.

Police said in a statement later that it received a call at about 5.30am requesting for assistance on the KJE towads Pan-Island Expressway.

Eight people were sent to hospital, they said, adding that three lanes were closed because one or both vehicles were leaking fluids onto the road. The road was passable to traffic from 8am, the statement said.

The Straits Times understands most of the accident victims - all men - suffered minor injuries, but one had "multiple fractures". They were sent to National University Hospital.

The accident took place even as calls for sterner enforcement action on heavy vehicles which flout traffic safety rules. Several ministers have weighed in on the issue, sparked by a horrific accident in Tampines on Monday, when two young brothers were mowed down by a cement truck as they crossed a traffic junction on their bicycle.