An injured migrant carries a child during clashes with Hungarian riot police at the border crossing with Serbia in Roszke, Hungary September 16, 2015.

U.N. Secretary-General Ban Ki-moon on Wednesday slammed Hungary’s treatment of migrants as unacceptable after police fired tear gas and water cannon at asylum-seekers at its border with Serbia.

“I was shocked to see how these refugees and migrants were treated. It’s not acceptable,” Ban told a news conference in response to a question about the border clashes.

The U.N. chief stressed that “these were people fleeing war and persecution” and that the response must be one of “compassionate leadership.”

“We must show them caring hands,” he said. “They must be treated with human dignity.”

On Wednesday, Hungarian police clashed for hours with hundreds of migrants after the government sealed its southern border with Serbia, one of the biggest entry points into the European Union.

Police fired at least 20 tear gas grenades as the crowd chanted slogans in Arabic with their fists in the air.

Children were crying from the effects of the tear gas and several ambulances were dispatched to the border crossing.

“It’s not a crime to cross a border,” the head of the U.N. refugee agency Antonio Guterres said in a statement, which warned that some refugee deterrence measures being implemented by Hungary violated international law.

Guterres demanded that Hungarian authorities “ensure unimpeded access for people in need of protection in line with its legal and moral obligations.”

The United Nations has made repeated appeals to European leaders to uphold human rights as the continent grapples with its worst migration crisis since World War II.

Europe’s migration crisis is expected to loom large over this year’s gathering of world leaders at the United Nations, which kicks off on September 25 with an address by Pope Francis.

Meanwhile, Hungary’s Prime Minister Viktor Orban said he hopes to build a new wall along his country’s border with Croatia to keep migrants out, he told French newspaper Le Figaro in an interview to be published Thursday.

After Hungary fenced off its border with Serbia in a bid to shut off a massive influx of refugees and migrants, Orban said people smugglers would simply change their routes and find new ways into the European Union.

“Since they can no longer pass through Hungary, they will change route and go through Romania, probably,” Orban told Le Figaro.

“That’s why we also decided to build a fence at the Romanian border, along the Mures River. And we will probably build another along the Croatian border. We are following their trail,” he said.

“The fact is the migrants keep coming. We managed to stop them at the Hungarian border, but this did not stop the influx itself.”

The Hungarian leader reiterated his staunch opposition to any bid to make EU members accept set quotas of refugees, vowing to do “everything I can to oppose it”.

Riot police in Hungary — which has received over 200,000 migrants this year, almost all of them heading to other EU countries — fired tear gas and sprayed water cannon on Wednesday at huge crowds of migrants desperate to cross the border from Serbia.

Also on Wednesday, the Romanian foreign ministry said it had summoned the Hungarian ambassador to express concern over Budapest’s unilateral decision to erect a fence on their shared frontier.