Prime Minister Lee Hsien Loong said he was saddened to learn of the accident involving two young brothers who were killed when their bicycle was hit by a cement mixer on Monday evening.

"So tragic that an evening cycle could end like this. My thoughts are with their family," he said in a Facebook post at about noon.

"We will render all the help they need to get through this difficult period. Meanwhile, let us continue to show sensitivity and respect for the family, especially online."

His post has since attracted more than 400 comments.

The two brothers, 13 and seven, were killed outside at the junction of Tampines Ave 9 and Tampines Street 45. Gory photos of the accident scene circulated online on Monday night.

A family member said that the mother usually picked up the younger boy from his school - Tampines North Primary - at 2.30pm. But that day, the boy had CCA which would end at about 5pm, and the mother had to work at 6pm.

The older brother volunteered to fetch the boy on his bicycle.

A 56-year-old Singaporean driver had been arrested for causing death by a rash act.