Undeterred by the death of three of their compatriots at the hands of US navy snipers, Somali pirates have hijacked two more cargo ships and opened fire on a third.

Despite tough talk from the Obama administration, the pirates – still acting with seeming impunity – launched a night raid on the Greek-owned MV Irene EM and seized the Togo-flagged MV Sea Horse. In a third incident, a gang fired automatic rifles and rocket-propelled grenades at the Liberian-flagged 21,887-tonne Safmarine Asia.

Nato officials expressed particular concern over the capture of the Irene, and the apparent new tactic of attack during darkness.

Lieutenant Commander Alexandre Fernandes said the Portuguese warship NRP Corte-Real received a distress call last night from the freighter Irene in the Gulf of Aden, where the west and China have deployed warships.

"There was only three minutes between the alarm and the hijack," he said. "[The pirates] attacked at night, which was very unusual. They were using the moonlight as it is still quite bright."

All 22 crew were unharmed, the East African Seafarers' Assistance Programme told Reuters. The night-time attack on the Irene suggests the pirates are using improved technology acquired from the proceeds of multimillion-dollar ransoms paid to release ships and their crews.

Hours later, officials on the NRP Corte-Real said a second ship, the nearly 5,000-tonne MV Sea Horse, had been seized about 77 nautical miles off Somalia. They said it was hijacked by pirates on board three or four skiffs, but had no other immediate details.

Yesterday other Somali pirates seized two Egyptian fishing boats off Somalia's north coast, according to Egypt's foreign ministry. The ministry cited a Somali diplomat in Cairo as saying there were 18 to 24 Egyptians on board the vessels.

Seven pirates have been killed in the last week by the US and French navies, but that seems to have had little deterrent effect.

On Sunday US naval commandos dramatically freed the American captain of the Maersk Alabama, who was being held hostage on a lifeboat.

Snipers, taking aim from the back of the destroyer USS Bainbridge, shot two of the pirates aboard the lifeboat. Only the heads and shoulders of the pirates were visible at the time, according to the US navy. A third pirate was also shot dead and one was captured.

A raid on Friday ended in a tragedy when Florent Lemaçon was killed as French commandos freed his yacht from a pirate gang. The 28-year-old Frenchman's wife, three-year-old son and two other passengers were successfully rescued.

Pirates contacted by telephone by news agencies have threatened retaliation, with one promising revenge against the next hostage they take from the US or France.

Admiral Michael Mullen, chairman of the US joint chiefs of staff, told ABC television that "we're very well prepared to deal with anything like that". The US president, Barack Obama, yesterday said America was determined to halt the rise of piracy in the region and would work with other countries to prevent future attacks.