There have been an estimated 10 hijackings by Somali pirates this month
Somali pirates have hijacked a fourth vessel in 48 hours, seizing a Lebanese-owned cargo ship.
The 5,000-tonne Togo-flagged MV Sea Horse was taken by gunmen in up to four skiffs, Nato officials said.
Earlier, pirates hijacked a Greek-owned bulk carrier, the MV Irene, in the Gulf of Aden a day after Somali raiders captured two Egyptian fishing boats.
Meanwhile, three Somali pirates who had taken French hostages were in custody in France, French prosecutors said.
The pirates were captured during a military operation to liberate hostages taken on the Tanit, a French yacht overtaken by hostages in the Gulf of Aden on 4 April.
The boat's French skipper and two other pirates were killed in the operation by French commandoes.
Analysts say the gangs are clearly not put off by that rescue or the liberation of a US skipper in operations that have left several bandits dead.
Somali pirate leaders - who have generally treated captives well in the hope of winning big ransom payouts - vowed on Monday to avenge the deaths.
Nato officials said another ship - the Liberian-flagged Safmarine Asia - escaped on Tuesday after coming under fire from pirates in several small boats.
After a lull earlier this year, the gangs have stepped up their attacks off Somalia's coast in recent weeks.
The 22 Filipino crew of the MV Irene, which was sailing from Jordan to India, are believed to be unhurt.
A Nato helicopter has reportedly been dispatched from a Canadian warship to investigate what is happening to the merchant vessel, flagged in Saint-Vincent and the Grenadines.
Nato Lt Cdr Alexandre Fernandes told Reuters news agency: "There was only three minutes between the alarm and the hijack.
"They attacked at night, which was very unusual. They were using the moonlight as it's still quite bright."
Hours later, Nato officials on a Portuguese warship announced the MV Sea Horse had also been seized off Somalia, although it was not immediately clear how many crew members were aboard.
Maersk Alabama crewman recounts ordeal
It is said to be the tenth hijacking in the Gulf of Aden and Indian Ocean since the start of the month.
Last Wednesday, a group of Somali pirates attempted to seize a US-flagged ship, the Maersk Alabama, but fled after taking its American captain hostage.
Following a stand-off with a US warship, three of the raiders were killed by three single shots from US snipers on Sunday and the skipper was freed.
Capt Richard Phillips and his crew are due to return to the US on Wednesday evening, the ship's owner told AP news agency.
Maersk Line Limited said they will take a chartered flight from Mombasa to Andrews Air Force Base in Maryland where they will be reunited with their loved ones.
'Peaks and troughs'
US President Barack Obama promised on Monday to "halt the rise of piracy" in one of the world's busiest shipping lanes.
Cdr Chris Davis, chief public affairs officer for Nato, told the BBC the surge in attacks could be simply down to chance.
"We do see peaks and troughs. Often weather-related and often it's just the situation as it arises and the opportunity - and that's what the pirates are, they are opportunistic."
Last Friday French forces killed two pirates and captured three more while freeing a yacht with five hostages, but the vessel's owner also died in the gunfight.
Shipping companies last year handed over about $80m (£54m) in ransom payments to Somali pirates.
The Horn of Africa nation has been without an effective government since 1991, fuelling the lawlessness which has allowed the pirates to thrive.
Efforts to stop the raiders have so far had only limited success, with international naval patrols struggling to cover the vast areas of ocean where the gangs operate.