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Islamic State group militants have recaptured a border town on the Turkish frontier after briefly losing the territory to Syrian rebels last week.

Heavy fighting has also taken place between Syrian opposition fighters and IS in Aleppo this week, and intensified in border areas after the Free Syrian Army took control of al-Rai on Thursday.

The rebels' offensive against IS has been backed by Syria's al-Qaeda franchise, the Nusra Front - and other militant groups.

Al-Rai's capture was a major boost for the rebels' war against IS, with the town having served as an important access point for supplies to the IS territories inside Aleppo province.

But as the rebel campaign against IS continues, opposition fighters could find themselves embroiled in a new conflict with the Syrian regime.

Syria's prime minister said on Sunday that the regime's military and Russian air force were planning a joint offensive against the rebels in Aleppo province.

Speaking with visiting Russian officials, Prime Minister Wael al-Hakali said plans were being made for the "liberation" of remaining opposition-held territories in Aleppo.

"We - together with our Russian partners - are preparing for an operation to liberate Aleppo and to block all illegal armed groups which have not joined or have broken the ceasefire deal," TASS news agency reported.

Russian upper house representative Dmitry Sablin confirmed "Russian aviation will help the Syrian army's ground offensive", .

A ceasefire between Syrian rebels and the regime was agreed in February, while Russia's air force announced a partial withdrawal of jet fighters in March. The truce does not include the Nusra Front or the Islamic State group, who both have a presence in Aleppo.

Since the ceasefore got off to a shaky start, battlefronts between regime and rebels have been subject to numerous ceasefire violations - but remain relatively peaceful compared with earlier this year.

Opposition districts continue to be blockaded by the regime, with new reports of starvation emerging.

Residents of Aleppo have also been subject to renewed barrel bombing by the regime's air force, after already suffering from heavy bombardments by Russian war planes.

Damascus launched a surprise offensive in Aleppo during peace talks with the opposition in February, and captured large parts of the province, cutting off an important supply route for the rebels.