A tanker and bulker have been boarded off Indonesia as gangs continue to target ships in the Singapore Strait.

The incidents happened within 15 minutes of each other on Sunday night, maritime intelligence company Ambrey Analytics said.

A Malaysian-flagged product carrier was boarded by six men four nautical miles (1.8 km) north of Kapalajernih Island, Indonesia, while heading eastbound in the narrow Philip Channel.

The suspects were reportedly armed with knives.

The tanker was heading to Port Dickinson in Malaysia and was able to continue its journey.

It is not clear if anything was stolen.

The tanker had an estimated freeboard of 2.3 metres and was underway at 7 knots.

Ambrey said that 15 minutes earlier, a Norwegian-flagged bulk carrier was boarded by a group of four raiders just two miles (3.2 km) away.

The ship was also transiting eastbound off Kapalajernih Island, heading from Port Klang in Malaysia to Fangcheng in China.

The gang was spotted with knives and gun-like objects.

The bulk carrier had an estimated freeboard of 5.1 metres and was underway at 9.6 knots.

The raids followed a boarding of a Greek-flagged tanker by armed men last Tuesday.

The unnamed vessel was targeted by two pirates carrying bladed weapons.

The latest two incidents bring the total to at least nine in the area since late December.

Most of the previous attacks have been on bulkers.

Ambrey says vessels with freeboards lower than 10 metres are at risk.

Crews are advised to lock down access to the accommodation block and ship’s stores and carry out deck patrols.