Syrian official media failed to report the loss of control of the headquarters of the 17th Brigade in the north-east of the country to the Islamic State (IS).
Meanwhile, opposition fighters shot down a helicopter in Aleppo on Sunday, and Free Syrian Army continued its advance in the middle of the country.
The official Syrian news agency, SANA, did not broadcast any news until late Sunday regarding the developments at the headquarters the 17th Brigade. IS took control of it after about a week since President Bashar Assad vowed to restore control there. In his oath speech, Assad said that he "will not forget to recover the beloved Raqqa" from the opposition, which seized it in March last year.
Pages loyal to the regime on the social networking sites published the photographs and names of dozens of officers and soldiers of regime troops killed in the 17th Brigade headquarters. Pro-regime activists spoke about a "tactical withdrawal" of regime forces, or said that the situation is "vague".
The death toll among regime troops killed in the attack and battles with the Islamic State in Raqqa province rose to more than 85 in two days, while 30 others were killed in an ambush set by IS in the countryside of Aleppo.
Supporters of the Islamic State broadcast on their accounts on the social networking sites videos from within the headquarters. They also published pictures of heavy military equipment said they seized from the 17th Brigade headquarters.
The Syrian Observatory for Human Rights said IS fighters did not station at the headquarters because of the "fear of regime's airstrikes". Activists have pointed to the withdrawal of some of regime forces from the nearby headquarters of Brigade 93 and a number of them were killed in an ambush set by IS.
Regime's forces have now lost one of its three remaining sites in Raqqa province which is entirely under the control of the IS. The other remaining sites are 93rd Brigade headquarters and the military airport in the city of Tabga in the west of the province.
28 fighters from the IS were killed in battles that accompanied the attack and bombing raids carried out by the regime, according to the Observatory.
The director of the Observatory, Rami Abdulrahman, said the Islamic State captured more than 50 soldiers after setting an ambush for them during their withdrawal from Brigade 17. Another 19 soldiers were killed in two suicide bombings at the start of the attack, while 16 others were killed in the battles which began Thursday.
Abdulrahman said that "hundreds of regime's forces withdrew on Friday to safe places in Brigade 93", adding that "the fate of some 200 soldiers is still unknown". He said that "the heads of dozens of regime's soldiers and officers were cut and their bodies were displayed on the sidewalks of the streets in the city of Raqqa".
In contrast, regime air forces launched raids on areas in Raqqa and the Observatory said that "five people, including a child, were killed as a result of a number of warplanes raid on areas near the National Hospital in the city of Raqqa".
IS launched on Thursday simultaneous attacks on the locations of the regime forces in the countryside of Raqqa and Hassakeh in the north east and in the countryside of Aleppo to the north.
These are the first major confrontations between the Islamic State and the regime since the start of the conflict. The director of the Observatory refers to IS desire to "clean" the areas under its control from the forces of both the regime and the opposition.
Translated and edited by The Syrian Observer