The Islamic State extremist organization Monday claimed responsibility for a suicide attack that killed some 32 Syrian rebels at a border crossing with Turkey the previous night.

The group's Aamaq Agency, quoting a "security source," said an Islamic State fighter blew himself up "among a gathering of members of the Free [Syrian] Army" at the Atmeh crossing.

The pro-opposition Syrian Observatory for Human Rights earlier reported that 32 people had been killed in a suicide attack on a bus carrying rebels from Turkey into Syria through the crossing some 40 kilometres west of Aleppo.

The rebels hit in the blast were rotating back into Syria to replace fighters returning from action, the Observatory said.

There were conflicting reports on the exact location of the attack. The Britain-based monitoring group said it took place on the Turkish side of the crossing and at least two Turkish border guards were also killed.

Turkish news agency DHA reported that the blast took place on the Syrian side and did not mention any casualties among Turkish personnel. It put the death toll at 35.

Syrian opposition outlet Orient News, quoting local activists, said the attack occurred outside a refugee camp on the Syrian side of the border.

Since the Syrian uprising started in 2011 the Syrian government has accused Turkey of using its border to facilitate the flow of rebel fighters into Syria.

The Britain-based Observatory has previously reported transfers of rebels from western Aleppo province and neighbouring Idlib province to rebel-held areas north of Aleppo via Turkish territory.

Rebels from Jaish al-Fath, a coalition dominated by hardline Islamist group Ahrar al-Sham and former al-Qaeda affiliate Jabhat Fatah al-Sham, control most of the territory on the Syrian side of the border.

In southern Aleppo, 35 rebels and 17 pro-government fighters were killed in fighting that broke out on Sunday as rebels tried to widen their area of control around a route that they recently forced through into the opposition-held eastern sector of the divided city, the Observatory said.

Government forces on Monday reversed rebel gains from their attack the previous day, while fighting also flared up again on the western edges of the city, the monitoring group said.

Meanwhile, Kurdish-led forces said they had lost 264 fighters in a two-month campaign, backed by intensive US-led airstrikes, that saw them wrest the strategic northern town of Minbij from Islamic State.

The Democratic Forces of Syria (DFS) on Sunday announced that they planned next to capture the town of al-Bab, west of Minbij.

That would cut Islamic State's territories off completely from the Turkish border, its last outlet to the outside world. It would also link up the main Kurdish-held territory in north-eastern Syria with the enclave of Efrin, north-west of Aleppo.

But the move also risks inflaming relations with rival opposition factions and with Ankara, which is wary of the Kurds' links with the banned Kurdistan Workers' Party inside its territory.

Turkey's Foreign Minister Mevlut Cavusoglu said he expected the US to "keep its promise" and make Kurdish forces withdraw from Minbij to the eastern bank of the Euphrates river, state-run Anadolu Agency reported. The US has not confirmed making any such guarantees.

The US backs the DFS, which is dominated by the Kurdish People's Protection Units (YPG), as the most effective force against Islamic State on the ground in Syria.

But the Kurdish movement is viewed with suspicion by the Syrian opposition, which accuses it of complicity with the regime of President Bashar al-Assad - a charge it denies.

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