By RADUL RADOVANOVIC 

OPATOVAC, Croatia (AP) — Thousands of migrants seeking to reach Western Europe were stranded in fog and cold weather in Croatia and Serbia on Sunday, a day after Hungary closed its border with Croatia and the flow of people was redirected to a much slower route via Slovenia.

Tiny Slovenia has said it will only take in 2,500 people a day, significantly stalling the movement of people across the Balkans as they flee wars and poverty in countries in the Middle East, Asia and Africa. On Saturday, over 6,000 people reached Croatia, and most of them were stuck in the country on Sunday.

The migrant route switched to Slovenia early Saturday after Hungary's right-wing government closed its border to Croatia for the influx, citing security concerns and saying it wants to protect the European Union from an uncontrolled flow of people.

Croatian police said Sunday nearly 4,000 people, including women holding babies in their arms, remained in the refugee camp in Opatovac, eastern Croatia, where buses and trains were waiting to take them toward Slovenia, the next step on their journey toward richer EU states, such as Germany or Sweden.

Across the border in Serbia, thousands of people have been sitting in some 50 buses since early hours Sunday waiting to cross to Croatia. More are expected to arrive during the day.

"We are waiting here 4 hours on the bus," said Muhammad Samin from Afghanistan. "The weather is too cold. We wear lots of shirts. The children are also in the cold. No food."

The United Nations refugee agency warned that Hungary's decision to close its border for migrants has increased their suffering and could lead to a backlog down the so-called Balkan route that goes from Turkey through Greece, Macedonia, Serbia and Croatia.

Babar Baloch, regional spokesman for Central Europe for the U.N. High Commissioner for Refugees, said the new migrant route through Slovenia has significantly prolonged their already weeks-long journey.

"The decision by Hungary to close its border has certainly added to the suffering and misery and the length of the journey for these desperate people," Baloch said. "There will be challenges if the process becomes slow or we have a backlog of people."

The Hungarian border closure is the latest demonstration of EU's uncoordinated response to the surge of people reaching its borders. More than 600,000 people, mostly Syrians, have reached Europe since the beginning of this year.

Hungary decided to close the border with Croatia after EU leaders last week failed to agree on a plan backed by Hungary to send EU forces to block migrants from reaching Greece from Turkey. It did the same on Sept. 15 on the border with Serbia after erecting a razor wire fence on both frontiers.

German Chancellor Angela Merkel was meeting Turkish leaders on Sunday to promote an EU plan that would offer aid and concessions to Turkey in exchange for measures to stem the mass movement of migrants into Europe.

Merkel arrived in Istanbul as thousands of new arrivals a day are stretching Germany's capacity to house refugees and other migrants.

Officials said the incentives offered to Turkey would involve an aid package of at least 3 billion euros ($3.4 billion) to help Turkey host the more than 2 million refugees that are in the country, as well as easier access to EU visas for Turkish citizens and re-energized EU membership talks.

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Dusan Stojanovic and Jovana Gec contributed from Belgrade, Serbia, Ali Zerdin from Ljubljana, Slovenia, Ivana Bzganovic, from Berkasovo, Serbia, and Amer Cohadzic from the Croatia-Slovenia border.