Ankara (AFP) - Islamic State jihadists were expelled from their last positions along the Turkish-Syrian border, Turkey said Sunday, as Syrian forces again laid seige to rebel strongholds in war-torn Aleppo.

Turkish Prime Minister Binali Yildirim said his nation's forces and Syrian rebels had pushed back "terrorist organisations" on its southern border with Syria, depriving IS of a key transit point for recruits and supplies.

"From Azaz to Jarabulus, our 91 km border has been completely secured," Yildirim said during a televised speech while visiting the southeastern city of Diyarbakir.

The news comes as Syrian government troops renewed the siege of rebel-held parts of Aleppo on Sunday as Washington and Moscow failed to reach a deal on stemming violence in the country's devastating war.

The more than five-year conflict has become increasingly complex, involving not only regime and rebels, but international backers on both sides, Kurdish forces, jihadists and now Turkey.

The Britain-based Syrian Observatory for Human Rights said earlier "rebels and Islamist factions backed by Turkish tanks and warplanes" had taken several villages on the Turkish-Syrian border "after IS withdrew from them, ending IS's presence... on the border."

Ankara began an operation inside Syria on August 24, using tanks and war planes to back opposition fighters with special forces also providing support.

Turkey's success is likely to deliver a blow to the Syrian Kurdish YPG, which has been gaining territory in Syria's north after working with the US-led coalition against the jihadist force.

But Ankara considers the YPG a "terrorist" group and has been alarmed by its expansion along the border, fearing the creation of a contiguous, semi-autonomous Kurdish region in northern Syria.

President Recep Tayyip Erdogan has said Turkey would not allow a "terror corridor" on its southern border.

With Turkey's rapid success in less than two weeks, his position looks stronger with territory in between the two Kurdish "cantons" of Afrin and Kobane now in the hands of Ankara-backed rebels.

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The loss of the Turkish border will also deprive IS of a key transit point for recruits and supplies, though the group continues to hold territory in both Syria and Iraq.

- Syria army besieges Aleppo -

Despite several rounds of international negotiations, a solution to the civil war that has killed more than 290,000 people and displaced millions remains elusive.

Syrian state media said the army and allied forces had taken an area south of Aleppo, severing the sole route left into the eastern neighbourhoods held by the opposition.

"The armed forces in cooperation with their allies took full control of the military academy zone south of Aleppo and are clearing the remaining terrorists from the area," state television said, citing a military source.

It said the advance "cut all the supply and movement routes for terrorist groups from southern Aleppo province to the eastern neighbourhoods and Ramussa."

The development leaves about 250,000 people living in rebel-controlled parts of the city cut off from the outside world once again, and will raise new fears about a humanitarian crisis in Aleppo.

Once Syria's economic powerhouse, the city has been ravaged by the war that began with anti-government protests in March 2011.

It has been roughly divided between government control in the west and rebel control in the east since mid-2012, but in recent months regime forces slowly began to encircle the east.

In July, they severed the only road into the rebel neighbourhoods, the key Castello Road running from the Turkish border in the north, creating food and fuel shortages in the east.

The siege prompted international concern, with aid agencies urging 48-hour ceasefires to ensure humanitarian access.

- US-Russia talks stumble -

In early August, rebel forces including Al-Qaeda's former Syrian affiliate battled regime forces south of the city to open a new route to the east, through Ramussa district.

But in recent days regime forces backed by Syrian and Russian war planes launched a counter-offensive.

A key regime ally, Moscow began an aerial campaign in support of President Bashar al-Assad's government last September, even as it continued to publicly support efforts for a negotiated solution to the five-year war.

Earlier Sunday, hopes were raised that Moscow and Washington might be on the verge of announcing a deal to halt the bloodshed.

US President Barack Obama said both nations were working "around the clock" on a ceasefire, and a State Department official said a deal was close.

But the hopes evaporated later in the day, with a State Department official saying Russia had "walked back on some of the areas we thought we were agreed on."

Instead, US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov are set to meet again on Monday in Hangzhou, China, where G20 leaders are gathered.

"We're going to review some ideas tonight, a couple things on these couple of tough issues, and come back together and see where we are," said Kerry.

"We're not going to rush," he said, stressing the importance of reaching a deal that was able "to try to get the job done".