Nine people have been arrested in connection with Wednesday's gun attack in Tunis that left 23 people dead, including 20 foreign tourists, the Tunisian presidency says.

It alleged that four were directly linked to the attack and five had "ties to the cell".

The army will also be deployed to major cities, the presidency added.

In another development, Islamic State said in an audio message that it was behind the attack.

It said the attack was carried out by "two knights of the caliphate" and named them as Abu-Zakariya al-Tunisi and Abu-Anas al-Tunisi.

A statement described the attack as a "blessed invasion of one of the dens of infidels and vice in Muslim Tunisia".

The statement was published by Twitter accounts known to be reliable sources of IS propaganda.

One of two gunmen involved in the Bardo museum attack, named by Tunisian officials as Yassine Laabidi, was reportedly known to the authorities.

Laabidi and his accomplice, named as Hatem Khachnaoui, were killed as security forces stormed the museum.

It was not immediately clear how the identities of the gunmen corresponded to the names given by IS. Jihadist groups, including IS, often give their fighters noms de guerre.

Tourists from Japan, Colombia, the UK and other European countries were killed in the attack and more than 40 people were injured.

The suspects arrested on Thursday were not identified and no further details of their alleged involvement were given.

However, BBC security correspondent Frank Gardner says an al-Qaeda operative has claimed that the two gunmen had spent two months training with Islamist militants in Derna, eastern Libya, before slipping back into Tunisia.

In a statement, the presidency said Tunisia was facing "exceptional circumstances", adding that "terrorist operations have now moved from the mountains to the cities".

"After a meeting with the armed forces, the president has decided large cities will be secured by the army," the statement added.
Stopovers suspended

On Thursday, two Spanish tourists and a Tunisian museum worker were found at the museum after having hidden there overnight believing the attack might not have been over, police said.

The attack is a huge blow for Tunisia's tourism industry and its government, which only emerged at the end of a long political transition several months ago, the BBC's Arab affairs editor Sebastian Usher says.

Two cruise companies - MSC Cruises and Costa Cruises - said they were suspending stopovers in Tunis.

MSC said in a statement that at least nine of those killed had been passengers on its MSC Splendida cruise ship which was docked in Tunis. It said another 12 of its passengers were injured and six were still unaccounted for.

Costa said that three passengers from the Costa Fascinosa had died. Eight others were injured and two were unaccounted for, company CEO Michael Thamm said in a statement.

Earlier, Tunisian Prime Minister Habib Essid told RTL Radio that security services had flagged up Yassine Laabidi but were not aware of "anything specific" or of any links to known militant groups.

In a televised address, Tunisia's President Beji Caid Essebsi said the country would not be cowed by terror attacks.

"These monstrous minorities do not frighten us," he said. "We will resist them until the deepest end without mercy."

The gunmen were killed after holding tourists hostage for several hours at the museum which is a major tourist attraction.

At the time of the attack, deputies in the neighbouring parliament building were discussing anti-terrorism legislation.

Sayida Ounissi, an MP, told BBC Radio Four's Today programme that according to the security services the gunmen had originally planned to attack parliament.

A statement released by a jihadist media outlet gave a similar account, saying the gunmen began killing tourists after being repelled by police at the parliament. The statement did not say which group carried out the attack.

Tunisia began a transition to democracy in 2011 with the overthrow of long-serving ruler Zine al-Abidine Ben Ali.

Hardline Islamists have tried hard to derail the process and security concerns have increased as neighbouring Libya has become increasingly unstable.

A large number of Tunisians have also left to fight in Syria and Iraq, triggering worries that returning militants could carry out attacks at home.

Many Tunisians took to the streets of central Tunis on Wednesday to protest against the museum attack, waving flags and lighting candles outside the museum.

Tunisia's Culture Minister, Latifa Lakhdar, told reporters that the museum would re-open to the public early next week.

BBC