European Union leaders have observed a minute's silence as they begin a summit overshadowed by the deadly attack on a Tunis museum.
"I invite everyone to observe a minute's silence in memory of the victims of the terrible terrorist attack in Tunis," EU president Donald Tusk said at the opening of the meeting in Brussels on Thursday.
Twenty foreign tourists, many of them European, were among those killed in Wednesday's attack on the national museum claimed by the Islamic State group.
Leaders of the 28-nation bloc expressed deep regret and condolences as they arrived for the summit in Brussels, deploring the attack as yet another example of terrorism coming closer to home.
French President Francois Hollande said the attack - which claimed two French lives - caused him "immense pain" and was doubly worrying because Tunisia had been making progress towards democracy and tolerance.
The Tunis killings have stirred memories of the January attacks by Islamist extremists in Paris which left 17 people dead and shootings in Belgium linked to fighters returning radicalised from the battlefields in Syria and Iraq.
The two-day EU summit is due to discuss the security situation in neighbouring Libya which has descended into chaos since the ouster of long-time dictator Muammar Gaddafi in 2011, stoking fears the Islamic State jihadist group could gain a foothold there and threaten Europe.
