Four people have been arrested for direct links to Wednesday's gun attack in Tunis that left 23 people dead, including 20 foreign tourists, the Tunisian presidency says.

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

One of two gunman involved in the Bardo museum attack, Yassine Laabidi, was reportedly known to the authorities.

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

The suspects arrested on Thursday were not identified and it was not clear what their alleged involvement might have been.

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

Tourists from Japan, Colombia, the UK and other European countries were killed in the attack.

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.