At least 88 civilians have been killed in 24 hours of Turkish air strikes on an Islamic State group bastion in northern Syria, a monitoring group said Friday.

A barrage of raids hit Al-Bab on Thursday, killing 72 civilians including 21 children, according to the Syrian Observatory for Human Rights.

Bombardment continued on Friday, leaving another 16 civilians dead, including three children.

"Eighty-eight civilians have been killed in 24 hours," Observatory head Rami Abdel Rahman told AFP.

He said it was the bloodiest attack by Turkish forces that his monitoring group had recorded since Ankara began its intervention in Syria in late August.

The Observatory says it determines whose planes carry out raids according to their type, location, flight patterns and the munitions involved.

Turkish forces and their Syrian rebel allies have been seeking to capture Al-Bab, about 25 kilometres (15 miles) from the northern Syrian border, for weeks.

On Thursday, IS released a video purportedly showing two captured Turkish soldiers being burned alive, after Ankara vowed to respond to 16 of its troops being killed in the fight against the jihadist group.

Turkish troops entered Syria on August 24 in support of pro-Ankara Syrian rebels, with the aim of ousting IS jihadists as well as Kurdish militia from the border area.

Turkish forces regularly carry out air strikes in support of the ground operation in Syria, but officials insist that the utmost is done to avoid civilian casualties.

Key dates since Russian intervention in Syria
Beirut (AFP) Dec 23, 2016 - Since late September 2015, Russia has staged air strikes in support of Syrian President Bashar al-Assad's forces, marking a major turning point in a multi-sided civil war.

It is the first time that Russia has pursued major military operations abroad since Soviet troops withdrew from Afghanistan in 1989.

Here is a timeline of the Syrian operation:

- Start of Russian intervention -

- September 30, 2015: Russia, at the Damascus regime's request, launches air strikes on Syria, saying it has hit Islamic State (IS) targets.

Russian President Vladimir Putin says Moscow must act pre-preemptively to destroy jihadists in Syria before they present a threat closer to home.

Syrian rebels and their overseas supporters accuse Moscow of targeting mostly non-jihadist groups to come to the aid of the Syrian army, which had suffered a series of resounding defeats since March 2015.

- Air strikes intensify -

- October 7, 2015: Moscow says that for the first time Russian warships in the Caspian Sea fleet joined in strikes in Syria with a volley of 26 cruise missile attacks against targets in Syria.

- December 9, 2015: Russia strikes Syria from a submarine deployed in the Mediterranean.

- Diplomatic offensive -

- October 20, 2015: Putin launches a diplomatic initiative and invites Assad to Moscow for a high-profile visit -- his first official trip abroad since Syria's conflict erupted in March 2011.

- From October 23, 2015: The US, Russian, Saudi and Turkish foreign ministers hold unprecedented talks in Vienna to try to find a way out of the Syrian crisis.

- November 14, 2015: After a first meeting on October 30, the big powers, including Russia, the United States, France, and for the first time Iran meet in Vienna where they agree on a fixed calendar for Syria but remain sharply at odds over the future of Assad.

- Regime forces retake territory -

- February 1, 2016: Syrian regime forces, backed by Russian air strikes, launch an offensive against rebels around the northern city of Aleppo. Rebels lose stronghold after stronghold.

- March 14, 2016: Putin announces a partial withdrawal of Moscow's forces from Syria which is widely publicised despite lack of details of how much of the contingent remains.

- June 18, 2016: Russian Defence Minister Sergei Shoigu meets with Assad in Damascus to discuss anti-terrorist "cooperation" and inspects Russia's air base in Hmeimim in the Syrian coastal province of Latakia.

- Russia-Turkey reconciliation -

- August 9, 2016: Turkish President Recep Tayyip Erdogan, whose country backs the Syrian rebels, meets Putin in Saint Petersburg in a bid to heal ties, strained by the Syria conflict.

Tensions had soared in November 2015 between the two rival players in the Syria war when Turkey shot down a Russian warplane on the Syrian border, saying it had violated its airspace.

- Permanent presence -

- August 16, 2016: Russia carries out strikes for the first time from the northwestern Iranian airbase at Hamedan.

- October 4, 2016: Moscow says it has deployed an S-300 advanced anti-air defence system at Tartus. An S-400 system was deployed in November 2015 at the Hmeimim airbase.

- October 14, 2016: Putin ratifies an agreement with Syria regarding the deployment "for an undetermined period" of Russian air forces at the Hmeimim base.

- November 15, 2016: Russia begins carrying out strikes from its only aircraft carrier, which is now in the Mediterranean.

- December 23, 2016: A day after the recapture of Aleppo by Syrian regime forces, Putin says Moscow will now "strive" to end fighting across the whole country. He calls Assad to congratulate him on Aleppo's "liberation". He signs an order to expand Russia's naval facility in Tartus.

Shoigu says Moscow has sent a battalion of military police to keep order in Aleppo.