Croatian Interior Minister Ranko Ostojic says his country will close its border with Serbia if another 8,000 migrants enter the country from Serbia in a single day.
Ostojic made the remark late on September 17 as thousands of migrants continue to flood into Croatia from Serbia – shifting their route toward Germany and other more prosperous European Union countries since Hungary sealed its border with Serbia on September 15.
Hundreds of migrants who have already reached Croatia broke away from police on September 17 in eastern Croatia’s border town of Tovarnik, setting off on foot down railway lines and through fields.
Some told reporters that the police wanted to transport them to refugee camps, but they want to continue traveling to Germany and other EU countries.
Ostojic said earlier on September 17 during a visit to Tovarnik that Croatia’s capacity to take in more migrants “is absolutely full.”
He called on other countries along the Balkan overland route for migrants – Greece, Macedonia, and Serbia – to “respect all agreements, all memorandums, all conventions” on migrants” and “keep people under international protection in their reception camps.”
Croatian police said on September 17 that more than 6,000 migrants had entered Croatia “around several crossing points" since the first groups started arriving early on the previous day.
Croatian authorities have been transporting the migrants to reception centers around the capital, Zagreb, after registering them.
Health Minister Sinisa Varga said Croatia expects an influx of more than 20,000 migrants over the next two weeks.
The influx puts Schengen member state Slovenia next in line on the most likely route to Austria and the chosen destination of most migrants, Germany.
Croatia has vowed to allow free passage of migrants seeking to reach northern Europe, but officials warned they could cope with several thousand migrants -- not tens of thousands.
Ahmed, a 22-year-old from Syria, says he nearly drowned while crossing the Mediterranean Sea in a boat to reach Greece before traveling on through Macedonia and Serbia, told RFE/RL’s Balkan service in Tovarnik that his goal is to reach Germany.
Ahmed said he wants to join his mother, who already is in Germany, and that he wants to study to work in the medical profession.
But he said he is scared because he does not know what will happen next, and also sad when he thinks about the difficult experiences he has already been through.
More than 500,000 migrants have entered EU member states this year, many of them escaping conflicts in Syria, Iraq, and Afghanistan.
The influx comes amid divisions within the 28-member bloc over how to deal with Europe’s biggest influx of migrants and asylum seekers since World War II.
WATCH: What Is The Schengen Area?
On September 16, migrants who had been prevented from crossing into Hungary hurled stones and other objects from the Serbian side of the border at Hungarian police.
Police responded by firing tear gas, pepper spray, and a water cannon across the border into Serbian territory.
Hungary said 20 police officers were injured as migrants tried to break through a gate.
United Nations Secretary-General Ban Ki-moon said he was "shocked" and said such treatment of asylum seekers was "unacceptable."
The UN's refugee agency, the UNHCR, said it is “extremely concerned” at violence on the border, which it said had injured several migrants and had caused children to be separated from their parents.
On a visit to the United States, Serbian Prime Minister Aleksandar Vucic condemned the "brutal treatment" of migrants.
WATCH: With Hungary Closed, Refugees Head To Croatia
"We will not allow anyone to humiliate us, and we will not allow anyone to throw tear gas on Serbia's territory," Vucic said.
Serbian Interior Minister Nebojsa Stefanovic said additional police were being sent to the border “to prevent further attacks on the Hungarian police from our territory and to separate in a humane and decent way migrants from the fences and the Hungarian police."
Hungary's foreign minister, Peter Szijjarto, responded to the criticism by saying the "bizarre and surprising" criticism of his country's police is "encouraging those people who are committing a crime."
Szijjarto said the migrants involved in the clashes are "aggressive people" who "will never be allowed to enter the territory of Hungary, not even for transiting purposes."
PHOTO GALLERY: Migrants Pour Into Croatia
Hungary closed its entire border with Serbia on September 15, making it a criminal offense to illegally enter the country or damage the fence it has built along its entire border with Serbia.
The country's courts have started trials of arrested migrants that could result in prison time or deportation.
Hungary says it now plans to extend its border fence along its shared frontiers with both Croatia and Romania to keep migrants from traveling around the border fence already built.
Romanian Prime Minister Victor Ponta responded to that announcement by saying that "the behavior of the Hungarian authorities has nothing to do with the idea of Europe. Fences, dogs, cops, and guns -- this looks like Europe of the 1930s."
Meanwhile, fellow EU member Bulgaria began deploying troops to its southeastern border with Turkey to avoid a refugee influx.
Interior Ministry official Georgy Kostov said the military could send up to 1,000 troops within the next week to back up border police if needed.
He said the move follows increased migrant pressure at the border overnight when 660 migrants attempted to cross into Bulgaria illegally.
Earlier, Austria began selective controls of vehicles entering from Hungary.
Germany has also started checking people at that country's borders with Austria and the Czech Republic and extended greater passport controls to its border with France.
Speaking after talks with Austrian Chancellor Werner Faymann in Ljubljana, Slovenian Prime Minister Miro Cerar said his country would stick to the rules of Europe's Schengen Area of border-free travel.
His government has said it will receive asylum requests but not create a "corridor" for refugees to simply pass through Slovenia and into Austria.
With reporting by RFE/RL's Balkan Service, Reuters, AFP, dpa, and AP