(Reuters) - Islamic State militants have executed Syrian army soldiers and are holding a group of them hostage after capturing an air base in northeast Syria at the weekend, pictures posted on the Internet and on Twitter by supporters showed on Wednesday.
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 studying 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. Reuters was not able to immediately confirm the authenticity of the images or when exactly they were taken.
Other photos showed groups of eight to 10 soldiers in fatigues taken hostage, some with facial wounds. Three are identified as officers. The photos appeared to show at least two dozen hostages.
One picture reportedly shows the body of a pilot who had appeared on Syrian state television before the attack on the base explaining how the army could easily defend it. Others show militants holding up knives next to groups of captured men.
Syrian state television aired a report last week interviewing army personnel at the base and showing its defenses, just before Islamic State overran it.
On Sunday, Syrian state television said that after fierce battles, the military was "regrouping" and that there was a "successful evacuation of the airport" as the army continued strikes on Islamic State in areas close to the base.
Raqqa is a stronghold of Islamic State, and some people celebrated in the city after the capture of the air base.
The Observatory said at least 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, making it one of the deadliest clashes between the two groups since the start of Syria's war.
The photos posted online also showed the attack on the base, which used at least one tank. Later pictures showed bodies on the ground and abandoned military hardware, such as a jet, warplane munitions and missiles, although it was not clear if any were operational.
Syria said on Monday it would cooperate in any international effort to fight Islamic State militants, but a White House spokesman said on Tuesday there was no plan to coordinate with Damascus on how to counter the threat.
President Barack Obama approved U.S. surveillance flights to gather intelligence on the extremist group after the release of a graphic video last week showing the beheading of a U.S. journalist by an Islamic State fighter.
(Reporting by Sylvia Westall; Editing by Hugh Lawson)
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