MADRID - The 16 Spanish crew members of a tuna trawler held for more than a month by Somali pirates arrived in Madrid on Saturday, the defence ministry said, as the skipper described being beaten during the ordeal.

The Alakrana crew members flew into a military base, a ministry spokeswoman said, after first travelling to the Seychelles in the Indian Ocean after their release on Tuesday.

Government officials and journalists were not allowed to witness their arrival at the request of family members.

The pirate group that hijacked the ship on October 2 claimed four million dollars (S$5.56m) had been paid for the release of the Alakrana and its 36 crew members.

It would be one of the highest ransoms ever paid since Somali piracy surged in 2007. The Spanish government has not confirmed it paid a ransom.

The other crew members included four Ghanaians, eight Indonesians, two Ivorians, two Madagascans, three Senegalese and one person from the Seychelles.

Ship skipper Ricardo Blach told Spanish media that "we were very badly treated -- the worst possible."

"They hit me, tied me up and 1,000 other things," he said, according to Spanish newspaper El Mundo.

Another crew member, Wilson Jean Pilate of the Seychelles, told El Pais newspaper that the pirates were heavily armed with Kalashnikovs and rocket launchers.

He said the pirates gathered the crew members at one point and shot in the air and water to frighten them.

Pirates seized the Alakrana when it was more than 300 nautical miles (550 kilometres) from the Somali coast, according to the ship's owners.

Spain in April allowed Spanish-flagged vessels to employ private security guards to protect against pirates off the coast of Somalia. Last month it also authorised the guards to use military-grade weapons.