Macedonian soldiers trudge through torrential rain in a borderland once home to quiet vineyards, searching for migrants trying to sneak through a hole in the locked-down gate to western Europe.

The voices of refugee children drift across from Greece, just the other side of barbed-wire topped fencing, where a multitude of colourful tents can be seen with clothes strung out on lines between them.

"The situation is hard for us as well as them. I see the children on the other side, I hear it's difficult. I myself have two children," said Marko, 34, one of the soldiers.

"But we have to do our job and protect our country."

Macedonia, a non-EU and non-NATO country of two million people, has deployed its army at the border since August last year to control the influx of people on the main refugee route through the Balkans.

But after countries further up the chain shut their doors to migrants over a week ago, Macedonia did the same -- and hundreds of its troops are now charged with keeping out at least 43,000 migrants massed in Greece.

More than 10,000 of them, many escaping war and poverty in the Middle East and Asia, are stuck in a border camp in increasingly poor conditions, with more arriving on the coastline of Greece -- both an EU and NATO member -- every day.

"It is incomprehensible to me that a small country like ours is the main defence of Europe, while Europe is not able to find somewhere for all those people who are suffering opposite us," said a 42-year-old soldier.

Like several others in the army who spoke to AFP, he did not want to be named.

- Desperate attempts -

Under a draft EU-Turkish swap deal, for every Syrian refugee sent back from Greece, the EU would resettle one from Turkish camps.

But with the controversial plan yet to be finalised, some migrants have made desperate bids to cross the Greek frontier.

On Monday, three Afghans drowned trying to wade through a river and into Macedonia, while another 1,500 or so who followed them made it across the border -- only to be rounded up and sent back by the troops.

"We are on duty around the clock. It's hard because we never know what could happen. When migrants rush to us, nervous, cold and hungry, tensions are high," said Milan, 29, from a mechanised infantry unit on guard.

The troops, who drove AFP along the muddy border in an armoured personnel carrier, can use their weapons only "when their life is in danger", according to army spokesman Toni Janevski.

But Macedonia's security forces have several times come under criticism for a heavy-handed approach at the frontier.

In February, Human Rights Watch accused border guards of "brutally" beating migrants as they tried to cross illegally, and police fired tear gas at hundreds who tried to break through the fence.

Some refugees said they were beaten after entering Macedonia on Monday, which officials strongly denied. The soldiers also accused the Greek side of encouraging the attempted crossings.

While the migrants remain in limbo, further confrontations seem likely, with the troops reporting an increase in illegal crossings and attempts to cut through the fencing since the Balkan route closed.

- 'Million-dollar question' -

Exactly what will happen next is "a million-dollar question and no one knows", Macedonia's Defence Minister Zoran Jolevski told AFP, calling for a "comprehensive strategy" that involves non-EU countries including Macedonia.

"I will be most happy if the situation is calm and we withdraw the army from the border fully, but it depends on the whole situation."

He said Macedonia's handling of the crisis showed it was a "credible partner of the international community" and one deserving of a place in NATO -- something Greece has blocked until now due to a dispute over Macedonia's name.

EU countries such as Slovenia and Hungary have sent police reinforcements to the border hotspot, while the EU in February approved 10 million euros ($11 million) to help Macedonia with the crisis.

But Macedonian President Gjorge Ivanov lashed out at Europe last week, telling Germany's Bild newspaper that his country had spent 25 million euros "paying for the EU's mistakes" and received "not a cent" in return.

While state leaders argue, troops at the border say they are increasing their patrols to cover terrain they had thought impassable until Monday.

"The migrants went through an area that's really difficult to access. We still wonder how they did it," said Janevski.