Serbia has banned Croatia’s defense minister from entering the country, a reciprocal measure against Croatia as tensions mount between the two Balkan rivals.

Serbia’s government on Thursday imposed the ban against Croatia’s Damir Krsticevic. Last week Zagreb said Serbia Defense Minister Aleksandar Vulin was not welcome.

A statement says the move was a “reciprocity measure.”

Last Sunday, Croatia accused Vulin of undermining its sovereignty with a statement in which he said Zagreb had no authority to decide on whether he could visit Croatia or not.

After Croatia’s move, Vulin said Zagreb’s barring of his visit to the memorial service in the village of Mlaka, near the site of the Jasenovac camp, was an attempt by Croatia, an EU member, to silence those speaking about the crimes committed there by the Nazi-backed Independent State of Croatia.

The spat further strains ties between the former war foes from the 1990s’ bloody breakup of the former Yugoslavia. A Croatian delegation earlier this month cut short a visit to Serbia over a far-right incident.

Croatia has said it banned Vulin because he declared that a decision on whether he would attend a Serb-organized World War II commemoration ceremony in Croatia depended on Serbia’s president, rather than Croatia.

Vulin is known for his anti-Croat stands. Hehas called Croatia’s decision an attempt “to silence those speaking about the crimes” committed at Jasenovac, where around 100,000 Serbs, Jews, Roma, and anti-fascist Croats were killed by the Ustashe regime, a quasi-protectorate under Fascist and Nazi patronage.

Relations between EU candidate Serbia and Croatia, which is a member of the bloc, have been strained since Croatia’s declaration of independence from the former Yugoslavia in 1991, which set off a four-year conflict with rebel ethnic Serbs supported by Belgrade.

On April 18, a Croatian parliamentary delegation cut short a visit to Belgrade after nationalist politician Vojislav Seselj, recently convicted of war crimes by a United Nations court, reportedly stamped on the Croatian national flag and cursed the visitors.