DAMASCUS (AFP) - Islamic State fighters have seized a Syrian army base in the northern province of Raqa, killing scores of troops and beheading some of them, a monitoring group said on Saturday.

The takeover of Division 17 base came as the UN Commission of Inquiry on Syria said on Friday that IS fighters accused of atrocities would be added to a list of war crimes indictees.

In a two-day assault on the base in Raqa province, an IS bastion, the jihadists killed at least 85 soldiers, said the Syrian Observatory for Human Rights.

More than 50 troops were summarily executed, 19 others were killed in a double suicide bombing and at least 16 more died in the assault launched early Thursday.

Hundreds of troops "withdrew on Friday to safe places - either to nearby villages whose residents oppose IS or to nearby Brigade 93 - but the fate of some 200 remains unknown," said Observatory head Rami Abdel Rahman.

"Some of the executed troops were beheaded, and their bodies and severed heads put on display in Raqa city," an IS stronghold, he told AFP.

Video shot by jihadists and distributed on YouTube showed IS fighters apparently inside Division 17 living quarters burning a portrait of Syrian President Bashar al-Assad.

The jihadists also distributed photographs via Twitter of bodies of decapitated soldiers strewn on the pavements and strung onto railings of Raqa city.

Mr Abdel Rahman said the public display was "a message to the people of Raqa, to tell them it is strong, that it is not going anywhere, and to terrify" any opponents.

Also in northern Syria, 30 troops and pro-regime paramilitary were killed in an overnight ambush in Aleppo province, the Observatory said.

IS, which first emerged in Syria's war in spring 2013, has since imposed its near-total control in Raqa province and Deir Ezzor on the Iraq border.

In June, the jihadist group proclaimed an Islamic "caliphate" straddling Syria and Iraq. Despite opposition by poorly-armed rebels fighting both the army and IS, the jihadists have made advances in several areas of Syria, whose three-year war has killed more than 170,000 people.

"There is a clear shift in the IS strategy. It has moved from consolidating its total control in areas under its grip. It is now spreading," said Mr Abdel Rahman.

"For IS, fighting the regime is not about bringing down Assad. It is about expanding its control," he said.

IS was emboldened by its June offensive in Iraq when large swathes of the north and west fell out of Iraqi government control.

Syrian rebels say IS transported a large amount of heavy weapons captured from fleeing Iraqi troops into Syria.

On Friday, Brazilian Paulo Pinheiro, who heads the UN Commission of Inquiry on Syria, said IS "are good candidates for the list" of possible war crime indictees.

"I can assure you that we are collecting information on perpetrators from all sides," he told reporters in New York.

The Observatory also reported that six children and three women were among 15 civilians killed Friday in rebel mortar fire on army-held areas of Aleppo city.

Regime air strikes, meanwhile, killed seven civilians, including three children, in opposition-held areas of Aleppo.

Once Syria's commercial capital, the northern metropolis has been divided into regime and rebel-held areas since July 2012.

Also in Aleppo province, a child was killed when a regime helicopter was shot down over the army-controlled Palestinian refugee camp of Nairab, the Observatory said.