Croatian President Kolinda Grabar-Kitarovic and Prime Minister Tihomir Oreskovic on Friday condemned the terrorist attack in Nice and expressed their condolences to French President Francois Hollande.
At least 84 people have died, including children, after a lorry slammed through a crowd celebrating Bastille Day in the southern French city of Nice on Thursday.
The driver ploughed on for 2km on the Promenade des Anglais at about 23:00 local time, before being shot dead by police.
Witnesses say the speeding lorry swerved and zigzagged in an apparent attempt to hit more people. Police reportedly found guns and grenades inside the lorry.
President Francois Hollande said the attack was of "an undeniable terrorist nature".
Prime Minister Manuel Valls has declared three days of national mourning for the victims from Saturday.
At the start of her address at the 11th Asia-Europe Meeting (ASEM) in Ulaanbaatar, Grabar-Kitarovic said that "in situations like this we mist stick together and we must maintain unity and commitment in the continuation of the determined fight against terrorism."
Prime Minister Oreskovic described the attack in Nice as monstrous. "I was deeply saddened by the news of yet another horrific terrorist attack in France. I most harshly condemn violence and terrorism which once again are taking human lives and causing pain to many families," Oreskovic said.
A state of emergency, in place since November's Paris attacks carried out by militant from the Islamic State group, in which 130 people died, has been extended by three months.