The visit was part of a renewed effort by the two countries that began in February to improve relations that have been sour since Croatia fought a war of independence from Yugoslavia in the early 1990s in which around 20,000 people were killed.

Head of the Serbian Radical Party (SRS), Vojislav Šešelj, grabbed a flag in a hall outside the parliamentary chamber in Belgrade, threw it on the floor, stamped on it, and tried to tear it up all the while hurling insults, witnesses said.

Security guards unsuccessfully tried to take away the flag. The Croatian delegates led by Gordan Jandroković, who is speaker of parliament in Zagreb, looked on. Another SRS deputy joined Šešelj in the incident.

“Because of this act which severely damaged the dignity of the Republic of Croatia … we have terminated the visit,” Croatia’s parliament said in a statement. The two-day trip started was due to end on Thursday.

Šešelj, a former Marxist, founded SRS in the early 1990s and was deputy prime minister under Slobodan Milošević during wars in the Balkans, in which more than 100,000 people were killed.

The UN war crimes tribunal for the former Yugoslavia last week overturned Šešelj’s previous acquittal on charges of persecution and inhumane acts for fomenting violence during the war.

He had spent 10 years in jail in pre-trial detention and was released in 2014 so he could return to Serbia for cancer treatment.

Leading Serbian officials, including Prime Minister Ana Brnabić and parliament speaker Maja Gojković, condemned the incident.

“Disrespect of state symbols of Croatia … is inadmissible and also represents an insult for the Serbian nation,” Gojković said in a statement. She once belonged to the SRS.

“I want to unequivocally, directly, and in the severest terms condemn Šešelj’s behavior,” Brnabić said in a statement.

Croatia joined the European Union in 2013, and earlier on Wednesday, Jandroković said Zagreb was ready to help Serbia with its own membership bid.