Tokyo: "No matter what the reasons were, terrorism can never be forgiven. We strongly condemn this," he said, adding that Tokyo was still working to collect information.
"We will exert the utmost effort in our fight against terrorism by deepening cooperation with the international community."
Speaking to reporters earlier in the day, Foreign Minister Fumio Kishida labelled the attack a "despicable act of terrorism."
The assault on Wednesday raised fears for the birthplace of the Arab Spring.
The brazen daylight raid sparked panic at the nearby parliament and the National Bardo Museum itself, a magnet for the tourists who contribute so much to the economy.
Among the dead were five Japanese, four Italians, two Colombians and one each from Australia, France, Poland and Spain, the Tunisian prime minister said.
It was the second time Japan has become embroiled in Middle East violence this year after the execution-style murders of a journalist and his friend by Islamist extremists in January.
Japanese Prime Minister Shinzo Abe said today at least three nationals were killed when gunmen stormed Tunisia's national museum, in what his government condemned as a "despicable act of terrorism".
Tunisian Prime Minister Habib Essid had earlier said five Japanese were among the 17 foreign tourists killed in the attack.
"At the moment, we know three Japanese nationals are dead and three injured," Abe told reporters.
"We will exert the utmost effort in our fight against terrorism by deepening cooperation with the international community."
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The assault on Wednesday raised fears for the birthplace of the Arab Spring.
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Among the dead were five Japanese, four Italians, two Colombians and one each from Australia, France, Poland and Spain, the Tunisian prime minister said.
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