TUNIS (AFP) - Tunisia said two gunmen who killed 21 people at its national museum trained at a militant camp in Libya, as the country marked Independence Day in sombre mood on Friday.

The Islamic State in Iraq and Syria (ISIS) group claimed Wednesday's attack on foreign tourists in Tunis, the deadliest since Tunisia's 2011 revolution which sparked the Arab Spring regional uprisings.

The assailants "left the country illegally last December for Libya and they were able to train with weapons there," Secretary of State for Security Rafik Chelly told Tunisian television.

ISIS, which includes hundreds of Tunisians in its ranks, threatened more attacks in an audio message claiming the massacre at the National Bardo Museum massacre.

Authorities say as many as 3,000 Tunisians have gone to Iraq, Syria and Libya to join militant ranks, raising fears of returning battle-hardened militants plotting attacks.

Chelly named locations of several suspected training camps for Tunisians in violence-wracked Libya, including second city Benghazi and the militant stronghold of Derna.

President Beji Caid Essebsi's office said security forces arrested nine suspects - "four people directly linked to the (terrorist) operation and five suspected of having ties to the cell".

Amid international outrage over the attack on Tunisia - hailed as a rare success story of the Arab Spring - Essebsi was defiant.

"The process of implementing a democratic system is under way, well anchored," he told French television.

"We will never move backwards."

Essebsi was set to give a speech Friday to mark the anniversary of independence from France in 1956, amid muted celebrations.

HONEYMOONERS HID IN STOREROOM

The dead included four Italians, three Japanese, three French, two Spaniards, a Colombian, an Australian-Colombian, a British woman, a Belgian woman and a Pole, health ministry officials said.

Two other Spaniards, a honeymooning couple, hid in a storeroom listening as the massacre took place, the husband told Spanish national television.

"We hid in a storeroom and stayed there, listening to everything that was happening and waiting for it to be over," Juan Carlos Sanchez said.

A Tunisian policeman killed in the attack was buried on Thursday.

The attack was a massive blow to Tunisia's economy, heavily tourism-dependent economy.

At least two major cruise ship operators suspended Tunis calls after the attack.

In Barcelona, hundreds disembarked from the MSC Splendida after their cruise was cut short.

Victoria Bronskaya, a tour guide from Belarus, recalled seeing two panic-stricken tourists gunned down as they tried to board the tour bus where she was cowering on the floor.

Their bodies blocked the way, perhaps saving her life because the "terrorist could not get on board".

Another passenger, Josefa Ramirez, said that after this experience "I don't want anything more to do with Arab countries".

Prime Minister Habib Essid named the two gunmen killed by security forces at the museum as Yassine Abidi and Hatem Khachnaoui, whose first name was later given as Jabeur by police in his home town.

Chelly said Abidi had been arrested before going to Libya, and that the pair had been "from sleeper cells".

Khachnaoui was described by a relative as a pious loner who never gave his parents a hard time. He was said to have abruptly dropped out of high school months before graduation and gone to Libya, and there are suspicions he also went to Iraq.

GUNMAN'S RELATIVES ARRESTED

Four of Khachnaoui's relatives have been arrested, the family and police said. Essebsi said the gunmen also had explosives and praised the security forces for preventing further bloodshed.

Abdelfattah Mourou, deputy speaker of parliament, told AFP guards supposed to be protecting parliament and the museum were having coffee as the assault happened.

"I found out there were only four policemen on security duty around the parliament (compound), two of whom were at the cafe. The third was having a snack and the fourth hadn't turned up," Mourou said.

A retired Colombian general whose wife and son were killed lamented the "irony" of losing them on holiday after spending his life "fighting terrorism" at home.

Tunisia has seen an upsurge in Islamist extremism since the 2011 revolution that ousted dictator Zine El Abidine Ben Ali and sparked the Arab Spring.

EU president Donald Tusk and foreign affairs chief Federica Mogherini will visit Tunis on March 31 to boost security cooperation, Mogherini said Friday.

Tunisia has taken pride in forming a democratic government and achieving stability since the Arab Spring - in marked contrast to countries such as Egypt and Libya.

But dozens of police and military personnel have been killed in attacks blamed on Islamist militants.