* Islamic State captured soldiers after seizing airport
* Tabqa airport had been last army foothold in area
* Sunni militants taunt Alawite soldier in video post (Adds other video, context)
By Sylvia Westall
BEIRUT, Aug 28 (Reuters) - Islamic State fighters have executed dozens of members of the Syrian army they captured after seizing an air base in the northeast of the country, a group monitoring the violence said on Thursday.
Islamic State, a radical offshoot of al Qaeda, stormed Tabqa air base on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the fiercest confrontations yet between the two sides.
The capture of Tabqa, the Syrian army's last foothold in that area, and apparent killing of large numbers of its soldiers shows Islamic State's grip on the north of the country. The group has also seized territory in eastern Syria and large areas of neighbouring Iraq in recent months.
The Syrian Observatory for Human Rights, which monitors the conflict using sources on the ground, said the soldiers who were executed had been trying to escape from the airport when they were captured by militant fighters of the Islamic State.
A video posted online by Islamic State supporters on Thursday appeared to show members of the group making scores of Syrian army captives walk and run through the desert in their underwear.
Reuters could not confirm the contents of the video, which was posted on You Tube and social media. It showed at least 135 men, some with their hands on their heads, running barefoot through a desert landscape as armed men jeered them.
It was not clear what happened to the men afterwards, but photos posted by Islamic State supporters online on Wednesday appeared to show them gunning down at least seven members of the army.
Another video posted online appeared to show the interrogation of at least one Syrian soldier in front of a group of other captured men in their underwear, as voices off camera shout sectarian insults.
The soldier identifies himself as an officer and says he is from the Alawite sect, an offshoot of Shi'ite Islam, like President Bashar al-Assad and the majority of high-ranking military officers. Islamic State members are Sunni Muslims.
"Who's your father? Do you know who your father is? You can't possibly know who your father is. You're a bastard," the interrogator says, using insults suggesting that Alawites are born out of wedlock.
At one point the soldier briefly looks down at the floor and rubs his eyes, another interrogator throws a metal rod at him, making him flinch and then pay attention to the main interrogator again.
"How many have you killed? How many have you raped?" the interrogator shouts. The soldier replies: "None. I've been stationed here in the airport."
The interrogator asks why the soldier had been fighting on behalf of Assad and did not defect and he replies that he would have just been sent back to the army.
"They would have sent you right back to the army? And we're going to send you right back to hell: by slaughter," the interrogator says, making him chant Islamic State slogans.
Syrian state media confirmed the attack on the base but has not reported any deaths or any army members being captured. It has said Islamic State suffered heavy losses in the battle over the base.
Tabqa was the army's last foothold in an area otherwise controlled by the Islamic State militants, who aim to set up a trans-border caliphate in the Iraqi and Syrian territory they have captured.
The United States has carried out air strikes on Islamic State in Iraq and has left open the option for similar action in Syria. (Additional reporting by Beirut bureau; Editing by Gareth Jones)