Islamic State has lost its last link between its main territories and the outside world as Syrian rebels backed by Turkish tanks and airstrikes captured the last stretch of the Syrian-Turkish border held by the extremist group.

Rebel forces advancing west from the town of Jarabulus and east from al-Rai captured the last border villages held by Islamic State on Sunday afternoon, according to the Syrian Observatory for Human Rights and rebel groups involved in the offensive.

Turkey's state-run Anadolu news agency confirmed the development, which puts an end to more than three years of Islamic State presence on the border.

The extremist group used the area, north-east of Aleppo, to bring foreign fighters into its territories in Iraq and Syria.

Its recruitment rocketed as it gained ground in the Syrian civil war from 2013 on and then captured swathes of northern and western Iraq in a lightning offensive in June 2014.

But it lost most of its border territory to Kurdish-led Syrian forces backed by US-led airstrikes who forced it from swathes of north-eastern Syria after defeating it in the battle for the border town of Kobane in 2015.

The US saw that campaign, spearheaded by the Kurdish People's Protection Units (YPG), as key to isolating the extremist group's de facto Syrian capital of al-Raqqa and disrupting its supply lines.

The jihadist organization's links to the outside world have now finally been severed by Turkey's first ground offensive inside Syria, backing rebel groups aligned with Ankara.

Turkey has said the offensive, which began ten days ago when Jarabulus fell to its rebel allies with little resistance, is aimed at both Islamic State and the YPG.

It launched the operation shortly after the YPG-led Syrian Democratic Forces captured the town of Minbij, south of Jarabulus, from Islamic State.

Turkey, which distrusts the Kurdish-led forces due to their links with banned Kurdish rebels operating on its own territory, feared that the capture of Minbij would lead to the Kurds advancing further west and linking up with an enclave they control in the far north-west of Syria.

That would have left Ankara facing a Kurdish entity spanning most of its southern border.

The Turkish-backed rebels last week clashed with Kurdish-aligned forces south of Jarabulus, driving them from a string of villages before calm was restored between the two US allies on Tuesday.

The Observatory, a Britain-based monitoring group, reported that the US envoy for the war against Islamic State, Brett McGurk, met Kurdish leaders inside Syria on Sunday and reassured them of Washington's continued support.

That meeting was not immediately confirmed by the US or by the Kurdish side.

Islamic State originated as the Islamic State of Iraq, an al-Qaeda affiliate which set up a Syrian branch called the al-Nusra Front after the Syrian conflict broke out in 2011.

Al-Nusra split with Islamic State in 2013, but many militants stayed loyal to the Iraqi leadership under Abu Bakr al-Baghdadi, who changed the group's name to "the Islamic State" following its victories in Iraq in 2014.

Unlike al-Nusra, which remains a close ally of other rebel groups, Islamic State fought all sides to gain full control of areas of Syria it dominated.

Its growing military strength, brutal killings of local and Western prisoners, and terrorist-style bombing atrocities at home and abroad quickly made its elimination a key objective for both Arab and Western leaders.

Islamic State is currently reeling under attacks from increasingly effective Iraqi security forces as well as the Syrian Democratic Forces.

The group on Wednesday announced that one of its top leaders and its official spokesman, Abu Mohammed al-Adnani, known for his calls for attacks on Western targets, was killed in the Aleppo area.

The US said it had targeted al-Adnani in an airstrike but could not confirm his death.

According to analysis firm IHS, Islamic State lost 12 per cent of its territory in the first six months of 2016 as well as suffering a drastic decline in its revenues.

The group still holds two key strongholds of al-Raqqa in Syria and Mosul in Iraq, while analysts have warned that territorial losses are likely to push the movement to intensify terrorist-style attacks such as those al-Adnani called for.

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