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Gunmen killed at least 19 people — including 17 foreign tourists — in an attack on the National Bardo Museum in Tunis, the capital of Tunisia, and their accomplices might still be at large, Prime Minister Habib Essid said Wednesday.

Essid said that two terrorists were killed in an operation to end the assault but that up to three possible accomplices could be on the run.

Describing the attack as "cowardly," Essid said at a news conference that the tourists were fired upon as they stepped off their bus to visit the museum near the North African nation's parliament. He said Polish, Italian, German and Spanish citizens were among the dead.

French President Francois Hollande later the deaths of two French citizens. He also said that seven French nationals were injured and another remained in serious condition.

My deepest sympathies to families of victims of Bardo museum attacks. EU stands united w #Tunisia against extremism. http://t.co/5BtPIz0hFP -Donald Tusk (@eucopresident) March 18, 2015

The attack is a blow for a country that relies heavily on tourism and has largely avoided major militant violence since its 2011 uprising to oust autocrat Zine El-Abidine Ben Ali.

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Efriqia Media, an independent pro-ISIS group that focuses on jihadist activities in Tunisia and Libya, identified the dead gunmen as Yaseen Al-Obaydi and Saber Al-Khashnawi. Laith Alkhouri, director of Middle East and North African research for Flashpoint, a global security firm and NBC News consultant, said Efriqia Media has previously released confirmed exclusive details and footage of jihadist operations.

Efriqia Media said the original plan of the gunmen, whom it called heroes, had been to target the Tunisian parliament, where the museum is located, and to kill everyone inside.

"I want the Tunisian people to understand once and for all that we are at war with the terrorists," Essid said in a nationally televised address late Wednesday. "We will resist them to the last breath without pity or mercy."

The U.S. condemned the "wanton violence" and praised Tunisia's response.

"We extend our heartfelt sympathy to the victims' families and loved ones," Secretary of State John Kerry said in a statement. "The United States stands with the Tunisian people at this difficult time and continues to support the Tunisian government's efforts to advance a secure, prosperous, and democratic Tunisia."

Footage from Tunisia's state-run broadcaster ERTT showed what appeared to be security services ushering dozens of civilians out of a street and into a building as the incident unfolded.

Poland's Foreign Ministry spokesman, Marcin Wojciechowski, said at a news conference that at least three Poles were injured. He said the wounded were part of a group of 36 Polish nationals touring the museum.

Italy's Foreign Ministry confirmed to NBC News that two Italians were injured and that 100 had been taken to safety. The nationalities of the other casualties weren't immediately clear.

The public affairs section of the U.S. Embassy in Tunis told NBC News it had no additional details and wasn't prepared to comment. A message later posted on the embassy's website warned of an "ongoing security situation" around the Bardo museum and urged U.S. citizens to avoid the vicinity.

French Prime Minister Manuel Valls said France was standing by its former protectorate.

"We are condemning this terrorist attack in the strongest terms," Valls told reporters in Brussels, according to Reuters. "We are very alert about how the situation is evolving."

M. Alex Johnson of NBC News contributed to this report.