ISTANBUL The border wends 800 km (500 miles) eastwards through hilly Turkish terrain, much of it mined. On squat concrete buildings across the valley, the Syrian flag flutters and soldiers clamber over rooftops watching for movement. Villages around are occupied by military.
The frontier between Turkey's Hatay province and Syria offers the probable site for the "humanitarian corridor" proposed by France to help civilians caught in the spiraling violence as President Bashar al-Assad fights to stay in power. But the Syrian military shows no sign of yielding control.
The French proposal is a work in progress. Turkish officials, whose government could be among the main actors, say they are still unsure what the proposal entails, and there were many scenarios.
"We talked extensively with the French about many options, what will happen if things get out of hand, if it deteriorates. Of course there are some back-up plans but we're not there yet," a Turkish government official told Reuters.
"It depends on whether the violence gets out of hand, if there is a coup, if there is a civil war that gets out of hand. There are many options and many things that could change the scenario," he added.
France's idea is for a corridor to provide access from frontiers such as Turkey and Lebanon or even to an airport where a plane could land or the coast where a ship could moor.
Aid agencies like the International Red Crescent would deliver relief and medical supplies to beleaguered population centers, and non-armed monitors could be placed with them to see that the Syrian authorities did not interfere.
For such a scheme to have any chance of success it would need Syria's agreement. The Arab League and foreign powers are seeking to persuade Damascus to accept such a scheme.
"So far they haven't said no, so we may be able to convince them," said a Western diplomatic source. "As long as we are in the humanitarian dimension it is harder for countries like Syria to refuse to allow aid to civilians."
Without Syrian agreement, the only way humanitarian corridors could work is if they were backed by force, ideally supported by a U.N. resolution.
"I think it would be very dangerous or difficult to try to have a humanitarian corridor inside Syria... I don't think Syria would accept that," commented Nikolaos van Dam, a former senior Dutch foreign ministry official and a historian of modern Syria.
Turkey's Deputy Prime Minister Bulent Arinc, in remarks on Thursday, gave a strong indication of Ankara's reluctance to be sucked into military involvement in Syria.
"Turkey intervening in Syria is completely wrong," Arinc told Turkish television channels. "As a country, we are not sending soldiers to Syria and we are not intervening."
Unlike Turkey's border with northern Iraq, where the Turkish army periodically mounts cross-border operations against Kurdish militants, the Syrian border is a very different prospect.
"The Syrians are patrolling the border. It's a very sensitive area. Syrians are in control there, not like in northern Iraq," van Dam said.
The Turkish army set up a security buffer zone inside northern Iraq during the first Gulf War in 1991 and have maintained small detachments there ever since. But their deployments were accepted by a Kurdish administration operating independently of Saddam Hussein's government in Baghdad.
WHICH WAY IN?
The Syrian army reinforced positions along the Turkish frontier in the early stages of the 8-month-old uprising against Assad's rule, as a previously friendly Turkey began running out of patience with Assad's repressive methods with the death toll mounting and thousands of Syrians fleeing across the border.
The Syrian army has manned watchtowers and established pickets on the hilltops overlooking the border, and soldiers have occupied the nearby mostly Sunni villages that were regarded as hotbeds of anti-Assad sympathizers.
Refugees and army deserters who have taken shelter in Turkey's southern province of Hatay tell of tanks dug in and camouflaged in the forested valleys south of the border there.
Turkey's border with Syria is some 800 km long, of which more than 500 km (300 miles) of the border is mined on the Turkish side to a depth of up to 300 meters, the legacy of an earlier era.
Though there are more than a dozen official border gates where vehicles can cross safely, most of Syria's cities are in the West, so the focus of any humanitarian operation has fallen on Hatay, which is hosting all five camps for refugees who have come over so far.
It is unlikely that any corridor would use the gate directly south leading to Latakia, a coastal city with a significant minority of Assad's own Alawite sect.
To reach the centers where Syria's majority Sunnis are more populous the route for any humanitarian corridor from Turkey's border would almost certainly pass through Aleppo, a largely Sunni and Kurd city.
Any corridor could then possibly run south down the M5 highway through a series of towns - Hama, Rastan, Homs - whose names have become synonymous with the bloody crackdown on pro-democracy protests. These towns are however closer to Lebanon.
Turkish foreign ministry officials briefing Turkish newspapers last week stressed opposition to foreign unilateral military action.
But they went on to say Turkey had contingency plans for either a no fly or buffer zone to protect Syrian civilians fleeing Assad's forces if there was a danger that Turkey could be inundated with refugees, as it was during the 1991 Gulf War, when half a million Iraqis fled to Turkey.
Turkey's attitude to military action could change if it thought there was a threat to its security, and President Abdullah Gul has warned Syria not to encourage Kurdish militant attacks in Turkey.
(Additional reporting by John Irish in Paris and Jonathon Burch, Ibon Villelabeitia and Tulay Karadeniz in Ankara; Writing by Simon Cameron-Moore)