Islamic radicals have seized most of an airbase in eastern Syria and are tightening a stranglehold on a minority Turkoman community north of Baghdad, as their pillage of the heartland of the Levant gains further momentum.

Militants from Islamic State (Isis) had breached the borders of the Tabqa base, which was the last base east of Aleppo still held by the Syrian regime, and now appeared certain to seize full control within days in another blow to all sides trying to slow the group's progress through Iraq and Syria.

About 300 miles to the east, a community of 18,000 Shia Turkomans holed up in the town of Amerli, north of Baghdad, said they were running out of food and hope after a two-month siege by Isis, which is trying to starve them to death or force them to convert to the militants' hardline brand of Sunni Islam.

In both locations, the bodies of fighters were reportedly defaced and displayed by Isis members, who have become feared and scorned for their unchecked barbarity.

Witnesses at the Tabqa base, not far east of the Isis stronghold of Raqqa, said that several captured Syrian soldiers had been beheaded.

Efforts to reinforce the base had failed, as had moves to evacuate up to 1,000 troops still stationed there as the extremists advanced.

Regime forces had fired at least one Scud missile, launched from near Damascus, at Isis forces nearby, but had failed to dislodge them.

In Amerli, surrounded residents said they were being left to their fate, unlike the Yazidis of north-west Iraq, who had escaped from Mount Sinjar after they were attacked by Isis.

Isis supporters celebrate in Tabqa. Photograph: Reuters

Turkish rebels who crossed from Syria and US air strikes secured an exit for the Yazidi refugees to Iraqi Kurdistan, but no such efforts have been proposed to assist the Turkomans.

Amerli is deep within an area in effect controlled by Isis. Kurdish peshmerga forces are stationed to the north, but they would be unlikely to mount a rescue without extensive air cover, which only the US or Iran could provide.

The US military has said it has hit more than 90 Isis targets since stepping in to try to slow an Isis advance towards the Kurdish north. More than 60 of those strikes targeted extremist positions near the Mosul Dam, allowing the peshmerga to recapture it from Isis last week.

However, Barack Obama has repeatedly signalled his reluctance to get further involved in Iraq, framing US involvement so far as protection of US interests and diplomatic staff in Irbil.

Debate has intensified in Washington in recent days between the Pentagon and the White House as the extent of Isis's rapid moves to the east and the west has become clear. The organisation appears to have shelved an attempt earlier in August to push towards Irbil and is now mobilising for an imminent advance through central Syria.

At the same time, Isis members are advancing towards the Turkish border in north-west Syria, attempting to secure a valuable supply line that has been the lifeblood of their growth so far.

The White House has come under growing pressure to attack Isis inside Syria, but officials are reluctant to do anything that would support the Syrian president, Bashar al-Assad, who they believe has tacitly supported the growth of the jihadists as a means of splintering the homegrown rebellion against him.

In the past six weeks, the Syrian air force has been used to attack Isis in the group's stronghold city of Raqqa and near the border with Iraq. However, close to Aleppo where Isis holds sway over the towns of al-Bab and Minbij, the group has not been attacked.