SINGAPORE (Kyodo) Pirates slipped aboard a Japanese-managed tanker in the South China Sea early Thursday, but there were no casualties and no damage, a Singapore-based piracy monitoring center said Friday.

Seven robbers armed with knives and short swords boarded the Panama-registered tanker High Energy from a small fishing boat just after midnight off the Indonesian island of Anambas, according to an incident alert from the ReCAAP Information Sharing Center.

"The robbers tried to enter the ship's cabin but failed as all the doors were locked. On hearing the alarm raised by the crew the robbers jumped overboard and escaped," the alert said.

Maritime sources said the 28,245-gross-ton tanker is managed by Nissho Shipping Co. and that a Japanese engineer was on board the ship at the time of the incident.

The tanker has a 22-member operational crew and was sailing from Kawasaki to Singapore when the incident occurred.