At least 25 civilians were killed in airstrikes by unidentified jets on Islamic State's stronghold in northern Syria, a monitoring group said Wednesday.

The strikes targeted al-Raqqa city, the de facto capital of Islamic State, late Tuesday, a day after the extremist group held off advances by Russian-backed Syrian regime forces south-west of the city, the Syrian Observatory for Human Rights said.

Six children were among the dead, according to the Britain-based Observatory, which relies on a network of activists inside Syria.

Syrian forces and allied militias, backed by Russian air power, launched an attack earlier this month aimed at cutting off al-Raqqa from the Turkish border.

The offensive came days after the Kurdish-led Democratic Forces of Syria, backed by US airstrikes, launched an attack targeting al-Raqqa, which has been under Islamic State control since 2014.

Elsewhere in northern Syria, unidentified warplanes struck rebel-held areas in Aleppo province, the Observatory said.

There were least 50 air raids since Wednesday morning targeting opposition-controlled areas in the north-western and northern countryside of Aleppo, the watchdog said.

The planes also bombarded the strategically important Castello road, which is held by the opposition and is the only supply route for rebels into Aleppo, leaving at least two civilians dead, the Observatory added.

Aleppo, Syria's pre-war commercial hub, is divided between a government-held western sector and the rebel-held east.

In recent weeks, fighting has surged in the city of Aleppo, shattering a truce brokered by the US and Russia that took effect in most parts of Syria in late February.

At least 666 civilians, including 134 children, have been killed in retaliatory attacks by the regime and rebels in Aleppo since April 22, when violence in the city escalated.

An air campaign launched by Russia in Syria in September has tipped the balance of fighting in favour of its ally President Bashar al-Assad.

In a sign of his strong hold on power, al-Assad on Wednesday appointed Emad Khamis as prime minister and tasked him with forming a new government, state news agency SANA reported.

Khamis, the electricity ministry in the outgoing government, is to replace Wael al-Halqi, who took the post in August 2012.

The West-backed opposition has repeatedly demanded al-Assad to step down in order to end Syria's five-year conflict.

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