UN Secretary-General Ban Ki-moon says he has been "shocked" to see how migrants are being treated after Hungarian police on the border with Serbia clashed with migrants.
Ban said he spoke by telephone on September 16 with Hungary's Prime Minister Viktor Orban about the treatment of migrants by Hungarian authorities.
His remarks came after migrants who have been prevented from crossing into Hungary hurled stones and bricks from the Serbian side of the border at Hungarian police on September 16.
The Hungarian police responded by firing tear gas, pepper spray, and water cannons across the border into Serbian territory. They described the angry crowd as "armed migrants."
The UN refugee agency, the UNHCR, said it was "extremely concerned" at violence on the border, which it said had injured several migrants and had caused children to be separated from their parents.
An injured migrant carries a child during clashes with Hungarian riot police at the border crossing with Serbia in Roszke on September 16.
Serbia's Prime Minister Aleksandar Vucic condemned what he described as the "brutal treatment" of migrants.
Serbian Interior Minister Nebojsa Stefanovic said additional police will be 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."
Meanwhile, Hungary announced on September 16 that it now planned to extend its border fence along its shared borders with both Croatia and Romania to keep migrants from traveling around the fence it has built since early August along its entire border with Serbia.
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."
WATCH: What Is The Schengen Area?
That announcement came after hundreds of migrants seeking to enter the EU began streaming into Croatia from Serbia after Hungary locked down its border.
Migrants arrived on September 16 by bus, car, and foot at the town of Sid in western Serbia and at nearby areas along Serbia's border with EU-member Croatia.
Police in Croatia said on September 17 that 5,650 migrants have entered the country so far. Croatia is transporting the migrants to reception centers around the capital, Zagreb, after registering them.
It said it could cope with several thousand migrants, but not tens of thousands.
Croatian Interior Minister Ranko Ostojic said on September 16 that new measures might be considered if the number of migrants grows.
As many migrants walked through fields and forests to cross the relatively unguarded border, Croatia sent deminers to certain border areas because of the danger posed by land mines left over from the Balkan wars of the 1990s.
There are at least five large areas in eastern Croatia -- where Croats fought against Serbian forces in the wars of Yugoslav succession -- that have an elevated risk for land mines.
One such area is along a stretch of Croatia's border that meets both Hungary and Serbia.
More than 500 people have been killed by land mines in Croatia during the past 20 years.
The wave of migrants began crossing from Serbia into Croatia after EU-member Hungary sealed its border with Serbia on September 15 and began enforcing new laws calling for prison terms of up to three years for those who breach Hungary's newly erected fence along its border with Serbia.
Hungary says that since September 15, it has arrested 519 migrants who succeeded in crossing the border. They have been charged with illegally entering the country.
There are still several thousand migrants poised on Serbia's side of the Serbian-Hungarian border.
Many others were traveling toward Croatia on September 16 -- hoping to cross that country and travel on through Slovenia to reach Austria and then Germany, the ultimate destination for many migrants.
Migrants Still Welcome?
Croatian Prime Minister Zoran Milanovic said the migrants would be registered as they enter Croatia.
"Croatia is entirely ready to receive or direct those people where they want to go, which is obviously Germany or Scandinavian countries," Milanovic told Croatia's parliament on September 16.
"It doesn't matter which color or faith they are," he said. "Those people are here, they are women, children, and men who want to live and create, but they don't want to be in Croatia."
Croatia is not part of the EU's passport-free Schengen area, so individuals traveling from there into Schengen neighbors Slovenia or Hungary may be required to present travel documents.
Croatian President Kolinda Grabar-Kitarovic on September 16 called for a meeting of the country's Security Council to discuss the migrant crisis.
"As the [migrant] crisis is getting more complex every day, I have to warn once again on the migrant wave and its possible social, economic, and security implications," Grabar-Kitarovic said.
She said the meeting could be held on September 18 at the earliest.
Slovenia's Prime Minister Miro Cerar said on September 16 that it will set up temporary border controls along its border with Hungary because of the migrant crisis.
Austrian officials have begun selective controls of vehicles entering from Hungary at three main border crossings.
Police said they may extend the controls to all 10 of Austria's border crossings with Hungary.
Austrian Interior Minister Johanna Mikl-Leitner has said that legitimate refugees from Syria and others at risk in their home countries can ask for asylum and are also free to travel to Germany.
But German security officials have also begun checking people at that country's borders with Austria, as Germany has been overwhelmed with refugees and other migrants the past few months.
More than 500,000 migrants have entered the European Union since the start of 2015 -- many of them escaping conflicts in Syria, Iraq, and Afghanistan.
Meanwhile, the Greek coast guard said on September 16 that it had picked up hundreds of people from small boats in the Mediterranean Sea near the Aegean Islands.
The coast guard said it had rescued 773 people in the past 24 hours and that many others had arrived on Greek islands by themselves.
Greek police added that some 5,000 migrants crossed the country's northern border into Macedonia in the 24 hours ending on the morning of September 16.
Turkish President Recep Tayyip Erdogan blamed the refugee crisis on Syrian President Bashar al-Assad, saying "the solution of this crisis is through bringing down the tyrannical regime in Syria."
But Erdogan -- who spoke with German Chancellor Angela Merkel about the crisis on September 16 -- also criticized European countries that build fences, close borders, and accept only Christian refugees.
Assad said in an interview with Russian television on September 16 that Europe is to blame for the migrant crisis because of what he claimed was the West's support for "extremists."
Assad accused Europe of providing "protection for terrorists [but] calling them moderates."
With reporting by Reuters, AP, and AFP