Updated

Pirates have hijacked a Danish sailboat with four adults and three children aboard as they were crossing the Indian Ocean, Denmark's government said Monday.

Most hostages captured in the pirate-infested waters off East Africa are professional sailors, not families. Pirates are not known to have captured children before.

The Danish Foreign Ministry said the ship sent a distress signal on Thursday. On board was a Danish couple, their three children — aged 12-16 — and two adult crew members, also Danes.

"It has now been confirmed that the sailboat was hijacked by pirates in the Indian Ocean," the ministry said in a statement.

Two days before the Danish sailboat sent the distress signal, four Americans were killed by Somali pirates in a hostage standoff. They were the first Americans slain by Somali pirates since a wave of attacks began six years ago.

Danish Foreign Minister Lene Espersen said she was "deeply concerned" about the situation and expressed her sympathies to the Danes on the boat and their relatives.

"It is almost unbearable to think that there are children involved and I can only sharply denounce the pirates' actions," Espersen said.

Government officials will do "everything in our power" to help the Danes, she said, but would not be able to release detailed information about their efforts publicly, "since experience shows that it doesn't help in resolving the case."

Danish news agency Ritzau, citing Foreign Ministry spokeswoman Charlotte Slente, said the boat was believed to be heading toward Somalia.

Earlier Monday, the European Union Naval Force said Somali pirates hijacked a Greek-owned cargo vessel with 23 crew on board.

The MV Dover was seized Monday in the north Arabian sea, 260 miles (420 kilometers) northeast of the Omani port of Salalah, the naval force said. The MV Dover was on its way to Yemen from Pakistan when it was attacked. It was registered with shipping and naval authorities.

There are three Romanians, 19 Filipinos and a Russian aboard the Panama-flagged vessel. There is no communication with the ship and no information regarding the condition of the crew.

In a separate incident, pirates released the MV Izumi on Friday, the naval force said Monday. The Panama-flagged vessel and its Filipino crew of 20 are believed to be making for a safe port. There is no information on the condition of the vessel or the crew. The ship was taken in October.

Somali pirates have extended their range east and south after increased naval patrols in the Gulf of Aden. They hold more than 660 hostages and some 30 vessels. If a vessel's owner is unable to pay the multimillion dollar ransoms the pirates demand, they may keep it and use it to stalk other vessels until they run out of supplies or break down.