Ships sailing in a key pinch point in the waters near Houthi-controlled Yemen reported spotting skiffs and a helicopter, both armed, not far from the latest Red Sea attack on shipping.

Maritime security firm Diaplous Group said on Tuesday that the helicopter may be accompanying the skiffs, though no attacks on shipping were reported.

The presence of such vessels in the Bab al-Mandeb Strait, which links the Gulf of Aden with the Red Sea, is certain to stoke concerns of a hijacking threat.

The report comes less than a day after a Houthi missile struck a Norwegian-controlled chemical tanker while it was in the Red Sea just 60 nautical miles (111 km) from the strait.

Diaplous reported that a Panama-flagged bulker reported that a warship in the area reported the presence of a helicopter armed with rocket launchers. The commercial vessel was not identified and tracking data shows an array of panamax bulkers are transiting the busy shipping corridor.

Later on Tuesday, a Cyprus-flagged product tanker reported seeing two skiffs in the same position. Armed security on the tanker reported seeing five people on each of the skiffs, which were of the type used in Yemen. The men were wearing military clothing, and the skiffs were equipped with rocket launchers, Diaplous said.

“It is likely that the helicopter is accompanying the skiffs,” the company said.

“All transiting vessels are advised to exercise extreme caution, report any suspicious activity and make a distress call to nearby warships.”

The sighting comes slightly more than two weeks after Houthi militants used a helicopter to storm Ray Car Carriers’ 5,100-ceu Galaxy Leader (built 2002) and take control of the ship, in an operation that bore the hallmarks of Iranian vessel seizures. The pure car/truck carrier re-emerged on vessel tracking data on Tuesday, showing it still in Houthi-controlled waters.

The hijacking marked the start of a series of attacks on vessels, with the Houthis vowing to target Israeli ships in response to the war on the militant group’s Hamas allies.

But Monday’s missile attack on J Ludwig Mowinckels’ 20,000-dwt chemical tanker Strinda (built 2006) represents a new tack by the Iranian-backed Houthis, which targeted a vessel without links to Israel other than a tentative deal to lift a cargo from the port of Ashdod. Its owner said it was actually en route to Italy.

Houthi military spokesperson Yahya Saree, who confirmed the group had fired on the tanker, said it would continue to block ships heading to Israeli ports.