Pirates attacked a South Korean-flagged cargo ship near the Singapore Strait early on Monday, assaulting the crew and making off with thousands of dollars in cash, Korea's Yonhap news agency said.

Singapore police, who manage the island nation's coastguard, did not comment immediately on the incident.

Piracy has fallen in the busy sea lane that runs past Singapore, Malaysia and Indonesia in recent years amid heavier policing.

However, the Yonhap report said seven pirates on a speedboat attacked the CK BlueBell before dawn, leaving some crew with minor injuries.