The European Union on Friday expressed concern at the expulsion of more than a dozen Croatian nationals from Serbia over their support for anti-corruption protests.

EU member Croatia said on Wednesday three of its nationals had been kicked out by its neighbour for security reasons in the previous days, following a dozen other similar cases in the past three months.

“We are concerned about those reports,” European Commission spokesman Guillaume Mercier told reporters in Brussels, adding Serbia had to abide by European and international human-rights standards.

“We’ll continue to monitor the situation and will not hesitate to raise those concerns with the Serbian authorities at the highest levels if need be.”

Some of those expelled had been living in Serbia for years and had voiced support on social networks for the student-led protests that have gripped the country since last year, according to Croatia’s ambassador to Serbia.

Among the latest group was the head of the Croatian Chamber of Commerce, he said.

The protests in Serbia, a candidate for EU membership, began in November last year, after 16 people were killed when the concrete roof of a newly renovated railway station in the northern city of Novi Sad collapsed.

High-ranking Serbian government officials have claimed, without providing evidence, that the protests were “influenced by Western intelligence agencies” with the aim of “overthrowing President Aleksandar Vucic”.

Ties between Belgrade and Zagreb remain frosty since Croatia’s 1990s war of independence, during which Serbia politically and militarily backed rebel ethnic Serbs.