Idomeni (Greece) (AFP) - Two hours queueing for a sandwich. It is a depressing waiting game for migrants on the Greek-Macedonian border -- and getting longer every day, as their mud-soaked camp grows in size, increasingly stretching worried aid groups' resources.

Among those stuck for days at the freezing Idomeni migrant camp is 25-year-old Iraqi Kadr Yussef, who recounts his last four meagre meals while hopelessly waiting for Macedonian authorities to let him through.

"Sandwich, sandwich, soup, orange," he says. Just enough to keep him going. As for his six-month-old daughter, she relies on milk handed out by aid groups.

Some 10,000 people mainly from Syria and Iraq have become trapped on the border after a spate of travel restrictions imposed by Balkan states on the migration trail to northern Europe, aimed at limiting a flow from the coasts of Turkey that shows no sign of slowing.

With some 25,000 migrants already in Greece, the government has struggled to provide accommodation. Healthcare and nourishment at the border have largely been left in the hands of aid groups and volunteers.

Most of the refugees have turned down the offer of relocating to camps in the hinterland, hoping to be nearby when Macedonian police briefly open the border for a few dozen people to cross each day.

The flood of people arriving at the border is relentlessly greater than the trickle allowed into Macedonia: between midnight and dawn Wednesday some 170 were let through after the border was opened for a few hours, having been closed since noon on Monday.

- 'Wondering how to cope' -

"Between Friday and Sunday the camp went from 4,000 to 8,000 people. Now it holds over 9,000," says Jean Nicolas Dangelser, the logistical supervisor of the Doctors Without Borders (MSF) charity.

"Each day we awaken wondering how we're going to manage," he adds.

Initially built to handle some 1,600 people, the camp has spilled over into the outlying fields, with thousands pitching tents on muddy soil.

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Aid groups have reported a shortage of food and tents, warning that the wet weather is taking a toll on the refugees and particularly their children.

MSF can provide some 6,500 warm meals but with cost overruns mounting, the organisation can then only offer snacks for the remainder of the day.

"Each meal costs 30-35 cents but our supplier cannot provide more," says Dangelser.

"Other suppliers charge around 1.2 euros per meal," he says.

- Food queue night and day -

The queue for food snakes for dozens of metres all day long, even though meals are only given three times daily.

And many refugees now go directly to where the food is prepared, snarling the procedure.

"One of our suppliers would prepare the food at the end of the camp and bring it in for distribution. But after people started going there directly as soon as the cooking began, they can no longer budge so the food is handed out there," says Dangelser.

MSF say they have help from an independent volunteer group that can prepare another 4,000-6,000 dishes of vegetable soup a day.

"They're a major help but the question remains, what do we do if we reach 20,000 refugees at Idomeni?" another MSF official said.

Two food trucks at the gates of the camp are making a killing with packs of biscuits at two euros ($2.17), French fry sandwiches at 2.5 euros and chicken sandwiches at three euros.

There is also a small supermarket at the village of Idomeni, some twenty minutes away, but many refugees know that they must save their money for the difficult road ahead.

Munir, a 33-year-old Syrian, finally succumbs to the aroma of the grill and walks away with a packet of crisps.

"I've been here a week and have no idea how much longer I'll have to stay," he tells AFP.

"I'm trying to spend as little as possible, he says. "This was already too much."