efe-epaBy Ilya U. Topper Suruc, Turkish-Syrian border

Turkey said on Saturday - the fourth day of its offensive against Kurdish militias in northeastern Syria - that it has taken control of the border town of Ras al-Ayn, marking what would be the first significant milestone of that military campaign.

Turkish television broadcast images provided by the armed forces that show soldiers and Syrian rebels allied with Ankara inside that town and in possession of a tank that had belonged to the People's Protection Units (YPG), the Kurdish militias that Turkey wants to expel from northern Syria.

Shells were fired throughout the day on Saturday from Ceylanpinar, a Turkish city just a few hundred meters from Ras al-Ayn.

Convoys of tanks headed toward the Turkish-Syrian border to bolster the offensive, while some pickup trucks carrying wounded combatants with the so-called Syrian National Army (formerly the Free Syrian Army), the new name under which Syrian militias allied with Ankara are fighting, moved in the opposite direction into Turkey.

The majority of these Islamist militia fighters had crossed into Turkey from western Syria and traveled through Turkish territory to Ceylanpinar to take part in the offensive.

The Turkish army also said it has advanced to highway M4, which is located around 20 kilometers from the border and traverses northern Syria from east to west.

That will allow the Turks to cut off a Kurdish supply route that runs from the de facto capital of Kurdish-controlled northern Syria, Qamishli, to Manbij, the bridgehead that the YPG have established west of the Euphrates River.

Lying in the middle of that supply route is the Kurdish city of Kobani, which in 2014 famously resisted a months-long siege by the Islamic State terror organization and where the sound of intense Turkish shelling was heard on Friday.

But there has been no ground invasion in that area, residents of Suruc, a majority-Kurdish Turkish city 10 km from the border, told EFE.

While Ras al-Ayn and especially Tal Abyad - the focal point of the Turkish offensive at this time - have significant Arab populations that are not always supportive of the YPG's presence, they said Kobani is a Kurdish city that will try to resist.

"The civilians now are all fleeing from Kobani. They're going to the small towns. All that's left in the city are the YPG militias to face off against the Turkish soldiers. They will fight," an electrician from Suruc told EFE on Saturday.

A small diplomatic incident erupted after it was learned that Turkish artillery fire had landed near a United States military observation post outside Kobani on Friday night.

Turkey's Defense Ministry confirmed Saturday that it had fired at a YPG position around 1,000 meters southwest of the US observation post. It denied that the US outpost had been affected but said Turkish troops ceased firing as a "precautionary measure" after the US had conveyed its concerns.

Besides the danger of being caught in the crossfire, residents of Suruc also expressed concern that the Turkish offensive will allow thousands of IS militants who had been jailed in Kurdish-held areas of northern Syria to escape and enter Turkey or even make their way to other European countries.

Suruc was the scene of a IS-claimed suicide attack in 2015 that left 35 dead during a meeting of Kurdish and Turkish leftist youth groups.

Following reports that five jailed Islamic extremists in northern Syria had broken out of prison, Turkey's Defense Ministry issued a statement Saturday saying it will do its utmost to maintain control over those prisoners and rehabilitate possible relatives of militants who do not have links to the Islamic State.

Turkey aims to carve out a so-called "safe zone" that will extend about 32 km into northern Syria, stretching from the Euphrates River to the Iraqi border.

That zone is intended for the resettlement of some two million Syrian refugees who took refuge in Turkey from their nation's eight-year civil war.

It also is to serve as a buffer between the YPG militias in Syria and members of the Kurdistan Workers' Party (PKK), which has mounted a smoldering insurrection against the state in mainly Kurdish southeastern Turkey for more than 30 years.

The Turkish army and allied factions of the Syrian National Army began the incursion into northern Syria on Wednesday in an operation dubbed "Peace Spring."

The YPG is the largest Kurdish militia in the region and was Washington's key ally in the fight against the Islamic State.

But Ankara considers YPG a terrorist organization indistinguishable from the Kurdistan Workers' Party (PKK).