The video, confirmed as genuine by an Islamic State fighter, showed the bodies of dozens of men lying face down wearing nothing but their underwear. They were stretched out in a line that appeared to be dozens of metres long. A separate pile of bodies was shown nearby. The authenticity of the video could not be independently verified.
The caption beneath it said the dead numbered 250. The Syrian Observatory for Human Rights, which monitors violence in the war, put the death toll at more than 120.
Islamic State stormed Tabqa airbase on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the bloodiest confrontations yet between the two sides.
The capture of Tabqa, the Syrian army's last foothold in the area, and apparent killing of large numbers of its soldiers shows how Islamic State has cemented its grip on Syria's north.
The video begins by showing the captives apparently being marched in the desert with their hands behind their heads. An Islamic State fighter repeatedly shouts out "Islamic State", to which the men reply "It shall remain".
Islamic State controls roughly a third of Syria, mostly areas in the north and east of the country. The United States has launched air strikes on the same group over the border in Iraq and is considering doing the same in Syria.
The Syrian government, shunned by the West, has presented itself as a partner in a war on Islamist extremists.
But Washington, which has built its Syria policy on President Bashar al-Assad leaving power, says he is part of the problem. French President Francois Hollande said yesterday that Assad was no ally in the fight against Islamic State.
Syrian forces on Wednesday lost further ground to other rebels, who seized control of the Syrian side of the sole crossing over the armistice line to the Israeli-occupied Golan Heights, a monitoring group said.
The takeover of the Quneitra crossing was led by al-Qaeda's Syria affiliate the al-Nusra Front, the Syrian Observatory for Human Rights said.
A group of 43 UN peacekeepers in the Golan Heights were detained by the militants yesterday, and the world body was working to secure their release, the United Nations said.