Pressure is building in Serbia and Hungary as officials brace for a new influx of refugees this week.
U.N. officials in Serbia said they expect 7,000 refugees to cross from Macedonia into southern Serbia in a 24-hour period between Tuesday and Wednesday.
To the north, near a gap in the fence on the Hungarian border where thousands have been crossing, tensions broke out late Tuesday as refugees, frustrated with having to wait for transportation, threw bottles at Hungarian police. Police responded by firing tear gas at the refugees.
Earlier in the day, refugees had broken past police lines at a holding center near the border fence.
Relief workers are struggling to keep up with the flow of refugees into northern Serbia, where refugees are arriving and waiting at makeshift camps while they decide how to cross the border into Hungary and travel on to the Austrian border.
Most said they ultimately want to go Germany, whose vice chancellor Tuesday said the country could take as many as 500,000 asylum seekers a year.
WATCH: Video footage of migrants on Serbian border
UNHCR team visits Serbian camp
One of the camps is at an abandoned brick factory on the outskirts of this town in Serbia's north. A team from the U.N. High Commissioner for Refugees visited Tuesday and found migrants sleeping amid garbage without enough drinking water, food or sanitary facilities.
"When the refugees and asylum seekers come to the north of Serbia, they already have had a long journey. They are tired and desperate to get to Hungary to continue their journey to the place where they want to go," said Niklas Stoerup Agerup, a UNHCR field officer visiting the camp for the first time Tuesday.
He said efforts are under way to boost the distribution of humanitarian supplies to the camp.
Until now, officials said they have focused relief efforts on southern Serbia, where private citizens have been delivering food to refugees camping in the capital, Belgrade, and other points south.
Phone support as a lifeline
Since Friday, Hungary has allowed refugees to go to the Austrian border. That has encouraged many to continue the trek north, in hopes of reaching Germany, where many have relatives or friends. Using mobile phones and social media, migrants have been communicating with relatives who are farther behind on the route to tell them about conditions and encourage them to continue the journey.
"We inform people about the options they have in Serbia," said Agerup. "If they choose to travel on, we don’t provide them with in-depth phones because people have smartphones."
Relief workers also know that the rules for crossing borders and transiting EU nations could change at any time. In Hungary, workers and prison laborers were busy Tuesday fortifying the fence along the Serbian border with razor wire. The tensions near the border crossing also raised questions about how much longer Hungary will continue to allow passage to the migrants.
Gripped by fear
Jalil Khalili, a 21-year-old Afghan who arrived penniless Monday at the abandoned brick factory in northern Serbia, hopes eventually to reach Britain, where he said his Pakistani wife is waiting for him. British authorities deported him to Afghanistan two years ago.
He stopped at the camp to rest and find the resources to keep going. With no blankets available, he said the cold kept him awake at night, as did the anxiety about the road ahead, even if he does make it all the way to Britain.
"I’m scared," Khalili said. "I’m scared of when I go back to London. Maybe they’re going to deport me again to Afghanistan."