HOYUK MILITARY OUTPOST, Turkey (Reuters) - Turkey is steadily sealing its frontier with Syria, long infiltrated in both directions by fighters and smugglers, with fences, minefields, ditches and a wall that will snake even through the most mountainous regions.

A Turkish soldier on an armoured military vehicle surveys the border line between Turkey and Syria, near the southeastern city of Kilis, Turkey, March 2, 2017. REUTERS/Murad Sezer

“The Border is Honor”, read signs across the walls of Turkish military outposts at Gulbaba and Hoyuk, visited by Reuters on a rare trip organized by the country’s armed forces.

Fortification of the 911 km (566 mile) border, along with a Turkish army incursion into northern Syria launched in August last year, is helping to tighten the noose on Islamic State fighters as well as curbing Kurdish rebel groups.

Rebels from a range of militias in the Syrian war, including foreign fighters joining Islamic State, once slipped easily over the border. The jihadist group also smuggled out goods including looted antiquities to raise funds for its struggle.

Now, with U.S.-backed rebels encircling its Syrian stronghold in the city of Raqqa, infiltration in either direction is no longer so straightforward. The clampdown has also sharply reduced the flow of Syrian refugees trying to flee the civil war.

“I can tell you that right now nobody with a vehicle or on horse can cross our border (illegally),” said infantry colonel Alparslan Kilinc, referring to the 169 km stretch from Hoyuk military post to the Turkish border town of Karkamis that his 1st Border Regiment patrols.

“It is just not possible. There are still attempts by people to cross on foot and we intervene in that.”

At Hoyuk, about 80 km northwest of the shattered Syrian city of Aleppo, Turkish soldiers demonstrate their readiness. One peers through binoculars towards Syrian territory from a watchtower at the perimeter of the small walled complex. Troops called to alert slide down a pole to the ground and run to a sandbagged position or mount an armored car.

Engineers are installing a complex set of measures across a territory that includes plains and mountains.

First comes a three-meter (10 foot) high wall, now almost complete, then a mined area. Beyond that lie ditches and fortified fences - an area patrolled by soldiers around the clock and monitored by thermal imaging cameras installed atop 25-metre high steel watchtowers to spot infiltrators at night.

Drones are also being used for surveillance.

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As a result, Kilinc said, the number of smuggling attempts, which peaked in 2014 at 3,474 incidents, dropped to just 77 last year. Illegal crossing attempts fell to 8,531 from more than 12,000 over the same period.

Many of those were likely refugees, even though camps have been set up for them on the Syrian side of the border. However, 424 non-Syrian citizens were captured in 2015, with the majority thought to be Islamic State fighters. Last year, that figure fell to 210, along with 49 militants from Kurdish militia.

FIGHTERS CUT OFF

Ankara was accused by some Western allies of being too slow to stop the flow of foreign fighters to Islamic State in Syria and Iraq in the early years of the jihadist group’s rise.

Turkey has rejected this, saying it needs greater intelligence sharing with allies to intercept would-be jihadists from the group, also known as ISIS. It has stepped up security and launched the military campaign in Syria, codenamed Euphrates Shield, to push Islamic State away from Turkish borders.

Sam Heller, Beirut-based fellow at The Century Foundation think tank, said the sealing of the border had been successful, but had taken time to get underway.

“It looks like the Turks have finally, successfully, closed their last stretch of border with ISIS,” he told Reuters. “They probably could have done it sooner, but this was something that was subject to other political calculations and considerations.”

For Ankara, the threat from Kurdish militias was at least as great as from Islamic State, if not more so, he added.

The Turkish campaign took the Syrian town of Jarablus on the Euphrates river, cleared Islamic State fighters from a roughly 100 km stretch of the border, and then moved south to al-Bab, a strategic town now all but secured.

Asked about the passage of foreign fighters over the frontier, Kilinc said: “It is almost non-existent. The people trying to cross through here were going to places like al-Bab before. Now those places are emptied.”

Kilinc’s stretch has been one of the hottest spots on the frontier, having neighbored Islamic State-held territory for several years until Euphrates Shield. Hoyuk remains a focus of tension, just 3 km from a town held by Kurdish YPG fighters.

There have been no clashes with the YPG so far involving troops stationed at the two outposts, another commander said, but his soldiers are always on alert.

Ankara regards the YPG as the Syrian arm of the Kurdistan Workers Party (PKK) that has been fighting for autonomy or secession inside Turkey since 1984. The United States and European Union also see the PKK as a terrorist organization.

However, YPG militia have emerged as an effective partner for Washington against Islamic State, putting the two NATO allies at odds, particularly over plans to retake Raqqa.

Turkey also wants the YPG to withdraw from Manbij, a Syrian town on the western banks of the Euphrates that the Kurdish militia has retaken from Islamic State. President Tayyip Erdogan said earlier this week that Manbij was the next target for Turkish-backed Syrian rebels.

As Turkey’s incursion is set to deepen, efforts to fortify defenses and security on the border are accelerated. Kilinc said the construction of 94 km of wall had been completed while the remaining 79 km on his stretch was expected to be finished within three months.

“Even the half-finished walls were good at deterring those who wanted to cross, making our job easier,” he said. “The external security of a country begins with good protection of its borders.”