By Ivana Sekularac

HORGOS, Serbia (Reuters) - Nila Eskanzadah, a 25-year-old midwife from Afghanistan, has been camping on the border between Hungary and Serbia for the past 16 days. She is on a list to cross to the other side of a razor wire fence into the EU free-travel zone on Thursday.

According to the United Nations High Commissioner for Refugees, the number of migrants in transit through Serbia has doubled to 1,800 this week from 900 a month ago.

While Balkan borders remain closed to migrants, aid workers warn that most of those 1,800 will put themselves in the hands of smugglers, while a few hundred, including Eskanzadah and her family, wait patiently to be allowed to enter Hungary legally.

Hungary lets 30 refugees into the country daily, mostly families from conflict zones.

"We don't have more money to pay to smugglers," Eskanzadah said as she opened a bag containing lunch provided by the UNHCR.

According to the Serbian government, 500 people, half of them children, are living in makeshift camps at two border crossings between Serbia and Hungary, in what are described as transit zones.

At the Horgos crossing where Eskanzadah and her family wait, there are 200 migrants, mainly from Afghanistan. They have access to one toilet and one drinking fountain.

UNHCR and the Red Cross distribute three meals a day, while Serbian police patrol the camps.

Tents, some made of sticks and blankets, have been put up along the wire fence Hungary put up last year to hold back the flow of migrants. Behind the tents is another metal fence to separate them from the highway to Budapest.

Mohamed Hakim Ayoobi, a 41-year-old from Kabul, says he has not had a shower since he arrived three days ago.

"I worked for Germans in Afghanistan and my life was threatened, that's why I had to leave," he told Reuters.

More than 650,000 people from the Middle East, Asia and Africa passed through Serbia last year on their way to the European Union, but in March the borders closed, stemming but not completely stopping the migrants.

So far this year Hungary says it has registered 19,140 asylum applications and more than 14,000 migrants have crossed its southern borders illegally.

Hungary passed a law on Monday that allows police to send illegal migrants detained within 8 km (5 miles) of its southern frontier back to Serbia.

"The approach is very restrictive and is not good. They (the asylum seekers) are left there in dire conditions," said Babar Baloch, UNHCR regional spokesman.

"This will only increase their suffering. They are trying to escape wars and persecution. We should try to help them, not make their lives more difficult."

(This story has been refiled to correct UNHCR regional spokesman's quote in paragraph 15.)

(Editing by Giles Elgood/Mark Heinrich)