The so-called Islamic State, an offshoot of al-Qaeda, stormed Tabqa air base near Raqqa city on Sunday after days of fighting with the army that cost more than 500 lives, according to monitoring group, the Syrian Observatory for Human Rights.

Tabqa was the army’s last foothold in an area otherwise controlled by the militants, who have seized large areas of Syria and Iraq. The United States has carried out air strikes on the group in Iraq and is considering its options in Syria.

In one picture posted online, a group of militants in balaclavas are seen gunning down at least seven kneeling men identified as army personnel. It was not possible to immediately confirm the authenticity of the images.

Other photos showed groups of eight to 10 soldiers taken hostage, some with face wounds and three identified as officers. The photos appeared to show at least two dozen hostages.

Other pictures show militants holding up knives next to groups of captured men.

The Observatory said 346 Islamic State fighters were killed and more than 170 members of the security forces had died in five days of fighting over the base, one of the deadliest clashes between the two groups since the start of the war.

The photos also showed the attack on the base, which used at least one tank. Other photos showed bodies and abandoned military hardware, such as a jet, warplane munitions and missiles, although it was not clear if any were operational.

Syria has been gripped by deadly violence since March 2011. According to the UN, over 190,000 people have been killed and millions displaced due to the violence fueled by the Western-backed militants.

The West and its regional allies, including Qatar, Saudi Arabia and Turkey, are giving financial and military support to the militants.

HH/HH