Karkamis (Turkey) (AFP) - Turkey sent more tanks into Syria on Thursday and sternly warned a Kurdish militia to withdraw from frontline positions, a day after pro-Ankara Syrian opposition fighters captured a key border town from jihadists.

The tanks joined those which crossed the frontier on Wednesday in the so-called Operation Euphrates Shield, which Turkey says aims to rid the northern Syrian border area of both Islamic State (IS) extremists and Kurdish militia.

Hundreds of Syrian rebel fighters -- backed by Turkish tanks, war planes and special forces -- had on Wednesday taken the town of Jarabulus to end over three years of jihadist control.

But Defence Minister Fikri Isik warned the Kurdish People's Protection Units (YPG) militia -- who also had designs on Jarabulus -- to move back east across the Euphrates or also face intervention from Turkey.

The new contingent of tanks roared across a dirt road west of the Turkish border town of Karkamis, throwing up a cloud of dust in their wake before crossing the border, an AFP photographer said.

The operation is the most ambitious launched by Turkey during the five-and-a-half-year Syria conflict and has been carried out in full coordination with its NATO ally the United States.

Jarabulus, a small town on the west bank of the Euphrates a couple of kilometres (miles) south of the border, had been held by IS jihadists since the summer of 2013.

- 'Up to 15,000 troops' -

It was not immediately clear if the deployment of the new tanks on Thursday was aimed at securing Jarabulus or helping the rebels move into new territory.

The well-connected columnist of the Hurriyet daily, Abdulkadir Selvi, said the aims of the operation included creating a security zone free of "terror groups" and limiting the advances of Kurdish militia.

He said 450 members of the Turkish military had been on the ground on the first day of the offensive but this number could rise to 15,000.

Turkey has emphasised that the offensive was also aimed at the YPG, who Ankara sees as a terror group bent on carving out an autonomous region in Syria.

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Ankara's hostility to the YPG puts it at loggerheads with its NATO ally, the United States, which works with the group on the ground in the fight against IS.

But US Vice President Joe Biden, visiting Turkey on Wednesday, made clear that Washington has strictly told the YPG not to move west of the Euphrates after recent advances or risk losing American support.

Turkey's defence minister told NTV television there was so far no evidence of any withdrawal and Turkey reserved the right to strike the YPG if it failed to move.

"If this withdrawal doesn't happen, Turkey has every right to intervene," Isik added.

"They have not yet withdrawn... Turkey will be following, moment by moment," Isik said, adding the withdrawal was promised within a week.

A spokesman for the US-led coalition against IS tweeted that the "main element" of the Syrian Kurdish forces had already moved east although some remained for clean-up operations.

- Selfies on the way -

Ankara has in the past been accused of turning a blind eye to the rise of IS but hardened its line in the wake of a string of attacks -- the latest a weekend bombing on a Kurdish wedding in the city of Gaziantep that left 54 people dead, many of them children.

Continuing a European tour in Sweden, Biden said there had been "a gradual mindset change in Turkey" to realise that IS was "an existential threat" to the country.

"I think the Turks are prepared to stay in the effort to take out ISIL (IS) as long as it takes," he added.

The Jarabulus operation proceeded at lightning speed with the town captured from IS just 14 hours after it was launched.

The speed of the advance stood in stark contrast to the long, grinding battles it had taken for Kurdish forces to recapture towns from IS in northern Syria, such as Kobane and Manbij.

Television pictures showed the Syrian fighters walking into an apparently deserted and abandoned Jarabulus unchallenged and newspapers published pictures showing that the rebels even had time to take selfies along the way.

The apparent efficiency of the operation also marked a major boost for the Turkish army whose reputation had been badly tarnished by the failed July 15 coup against Erdogan staged by rogue elements in the armed forces.

With the civil war that has killed an estimated, 290,000 people still raging, 11 children were killed on Thursday in a barrel bomb attack carried out by government forces on a rebel-held neighbourhood of Syria's Aleppo city, a monitor said.