A crude tanker has reported a suspicious approach by armed men on a small boat off Yemen after a weekend that saw Houthi rebels step up their attacks against shipping.

The incident occurred on Sunday afternoon, seven nautical miles (13 km) south-west of Ghurayrah, according to Ambrey Analytics.

The Panama-flagged tanker said in a VHF alert that it was transiting north along the Bab el-Mandeb strait at a speed of 12.6 knots when the incident took place.

Armed persons were spotted on board a small vessel that approached within six nautical miles.

The tanker’s estimated freeboard was 6.4 metres.

The crude carrier increased its speed.

“Vessels in the vicinity were advised to maintain watch for small boat activity and report any suspicious activity,” Ambrey said.

Naval and security sources reported two attacks on vessels by Houthi forces over the weekend.

The United Kingdom Maritime Trade Operations was the first authority to report the two separate incidents, in which “unknown projectiles” impacted the 5,700-dwt multipurpose vessel Norderney (built 2012) and 5,900-teu MSC Tavvishi (built 2000).

Both ships were said to be proceeding to their next port of call after suffering fires on board that caused no casualties among the crew.

Ambrey said the Antigua and Barbuda-flagged Norderney was heading south-west along the Gulf of Aden at a speed of 8.2 knots when the forward station was struck by a missile on Saturday. A second missile was sighted but did not hit the ship.

The security company also said that gunmen from small boats in the vicinity opened fire on the ship, which changed direction to port and increased speed to 10.5 knots.

A third Houthi missile targeted and hit the same vessel again on Sunday, as it was sailing 89 miles (143 km) south-west of Aden.