By Humeyra Pamuk

ELBEYLI, Turkey (Reuters) - Emerging into Turkish territory through a field of towering corn a stone's throw from the Syrian border, more than a dozen men make their way under scorching heat towards two white vans by the roadside, their engines running.

Carrying rucksacks and bags, these new arrivals appeared to have crossed illegally into Turkey from a part of Syria just a few hundred meters away controlled by Islamic State militants.

A man on a motorcycle, apparently co-ordinating the group's arrival, grows visibly nervous, spotting a Reuters correspondent and cameraman and warning them not to film.

"Are you journalists? Get the hell out of here now or I will turn your car upside down," he said. "Get lost or I will get your car stoned."

Turkey's armed forces may have sent additional soldiers and equipment to this region in recent weeks as fighting in northern Syria intensifies, but smuggling along a border which thousands of foreign fighters are thought to have crossed to join the militant ranks appears to be continuing.

Just a few minutes before the group emerged from the corn field, a woman and child among them, a military vehicle passed along this road, stretching between the sleepy village of Uckubbe and the border town of Elbeyli.

"They (smugglers) bring people through the corn fields because the corn provides cover for them. It makes them invisible," said an Elbeyli resident who gave his name only as Yusuf, saying he himself had transported goods and people between Turkey and Syria, dodging security patrols.

"There are mines on the border but there is a narrow pathway there that is cleared. I know where it is and ISIL fighters know it too," he said, using an acronym for Islamic State.

Foreigners from more than 80 nations including Turkey, Britain, parts of Europe, China and the United States have joined the ranks of Islamic State and other radical groups in Syria and Iraq in the past few years. Many crossed via Turkey.

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The government has stepped up efforts to halt the flow, banning 15,000 individuals from 98 countries from entering Turkey and deporting some 1,500 suspected of seeking to join the extremists. But its Western allies want it to do more.

"They're a NATO ally. They have a strong stake in things, in stability to their south. I believe they could do more along the border," U.S. Defense Secretary Ash Carter told the Senate Armed Services Committee last week.

Turkey says foreign intelligence agencies need to stop their nationals traveling to Turkey in the first place.

"NOBODY TO STOP THEM"

The reinforcements sent to the border in recent weeks were prompted by intensified fighting between Islamic State and Syrian rebel groups, as well as by the advances of Syrian Kurdish militias, whose gains Turkey fears could stoke separatist sentiment among its own Kurdish community.

Armored vehicles frequently patrol this part of the 900-km (560-mile) border. Tanks are positioned outside small military compounds, their guns trained on Syrian territory, while soldiers stand guard at newly-built watch posts.

Within their sight, the black and white flags of Islamic State fly over buildings on the outskirts of the Syrian town of Jarablus.

"ISIS would not attack the Turkish army. Why would they? There are so many Turks within the ranks of ISIS now. Especially the ones just across the border ... who I mostly deal with," said Yusuf, stood next to his run-down car, its seats covered in dirt and its windscreen cracked.

Turkish officials say there are less than 1,000 Turks among Islamic State's ranks, but Western diplomats say the figure could be far higher. Pictures of Turkish jihadists killed in fighting in Syria have regularly appeared on social media.

Last week, Turkish police detained dozens of people suspected of belonging to Islamic State in a series of raids, a day after U.S. officials visiting Ankara urged Turkey to redouble its efforts against the militants.

The Turkish army also said last week it had detained almost 800 people trying to cross illegally from Syria, including three suspected Islamic State militants.

But despite the tighter security, the porousness of the border and the proximity of Islamic State have left Turkish residents feeling insecure.

"We’re literally living in the backyard of ISIS," said Civan Ali, 74, a retired farmer from the village of Karanfilkoy, pointing at his barn, just meters from a barbed wire fence separating his land from Islamic State-controlled Syria.

"If one night they decide to come and cut our throats, there’s nobody around to stop them. The soldiers can’t even stop the smugglers, how could they possibly stop the militants."

(Editing by Nick Tattersall/Ruth Pitchford)