Croatia opened its border with Serbia for migrants Monday, letting in thousands who have been stranded for nearly two days as colder weather approaches, according to a United Nations refugee agency official.

Spokeswoman Melita Sunjic, who is on the border, said that "without any announcement, the borders opened. When the borders opened, everybody rushed" over.

Sunjic said "the last person to go was a young boy without a leg, and we helped him cross in a wheelchair."

There were between 2,000 and 3,000 migrants stuck on the border in mud and rain when the gates were opened.

Doctors have warned of medicine shortages for as many as 10,000 refugees stranded in Serbia, Sky News reports. Those doctors say insufficient aid could severely affect children suffering from hypothermia.

"We don't have a chance to treat, we don't have the actual medicine to be given out to them, we don't have any more rain coats,” Dr. Ramiz Momeni, London-based founder of the Humanitas Charity, told Sky News.

"There is a lack of food, lack of blankets, we are missing everything,” UN Refugee Agency spokeswoman Melita Sunjic also told the media outlet.

Meantime, Turkey's prime minister said Monday he won't allow his country to be treated as a "concentration camp" for refugees but is willing to work with EU countries to stem the illegal flow of migrants.

Ahmet Davutoglu made the comments one day after German Chancellor Angela Merkel held talks with Turkey's leaders on a European Union plan that would offer aid and concessions to Turkey in exchange for measures to halt the mass movement of migrants across Europe's borders.

Davutoglu told A Haber news channel in an interview that the sides had agreed to set up a system that would put illegal migration under check. He said: "No one should expect Turkey to turn into a concentration camp where all of the refugees are kept."

Charities and aid agencies have been struggling to cope with huge numbers of refugees fleeing war and poverty in the Middle East, Africa and Asia. 

The EU has agreed to a plan, resisted by Hungary and several other countries, to share the redistribution of 120,000 refugees among its members.

This is a small proportion of the 700,000 migrants the International Organization for Migration expects will reach Europe's borders this year.

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The Associated Press contributed to this report.