Islamic State extremists rampaging through Iraq have now turned their sights back towards Syria, where only a besieged airbase stands between the terror group and a rush for the Mediterranean coast that could split the country in two.

The attack on the Tabqa airbase in eastern Syria comes as Isis continues to move back towards areas it controlled north of Aleppo until February. Using weapons the group looted from abandoned Iraqi military bases, Isis has returned with a vengeance to the area, stunning regional powers with its rapid advances.

Less than three months after taking Iraq's second and fourth biggest cities, much of Anbar province and the Syrian border, the group is establishing itself with extraordinary speed as a regional power that will determine the fate of both countries. There are growing fears across the Middle East that no regional military can slow the group's momentum.

Isis now controls a swath of land slightly larger than the UK, from Aleppo to central Iraq, and holds sway over a population of at least four million people. The group's rapid ability to organise and consolidate continues to splinter a fractured body politic in Iraq and Syria and is fast causing ramifications for the broader Middle East.

"The Islamic State is now the most capable military power in the Middle East outside Israel," a senior regional diplomat said on Friday. "They can determine outcomes in a few days that the Syrian rebels took two years to influence. Their capacity is in sharp contrast to the Syrian regime, which is only able to fight one battle at a time and has to fight hard for every success.

"In the first two months of its life, the so-called Caliphate has achieved unparalleled success. It is in the process of creating foundations for substantial financial, military and political growth. It is the best equipped and most capable terror group in the world. It is unlike anything we have ever seen."

Isis has surrounded the Tabqa airbase in Raqqa province from all sides, trapping 800 to 1,000 Syrian troops and airmen who have used the base to attack mainstream opposition groups in the north of the country for the past two years.

Despite Isis gradually subsuming parts of the opposition in northern Syria, the Syrian regime had not attacked the terror group until it launched its offensive into Iraq on 10 June. Since then, Syrian jets have bombed 30 targets in Raqqa, which had been a command hub for Isis for the past 18 months.

"They did not bomb the [Isis] headquarters until June and even then only after it had been evacuated. We are all paying the price now," the Kurdish intelligence chief Masrour Barzani told the Guardian this month.

Syrian reinforcements have rushed to defend Tabqa, but doing so will be difficult. Every other regime facility in eastern Syria has fallen over the past year and there is little chance of holding Tabqa without external help.

If Tabqa falls, Isis will have a relatively clear run towards Syria's fourth city, Hama, around 300 miles to the west. Hama is within a contiguous strip linking Damascus to the coastal cities of Latakia and Tartous, which earlier in the Syrian civil war were designated a de facto rump state that could be defended by loyalists.

Of more immediate concern, though, is the fate of the country's second city, Aleppo, which is almost surrounded by regime forces who, with the help of a relentless barrel-bombing campaign in rebel-held eastern Aleppo, have pushed the mainstream opposition to the edge of defeat.

A revitalised Isis threatens to change that dynamic. Using armoured vehicles supplied by the US to the vanquished Iraqi army, Isis has taken 12 villages in the Aleppo countryside in the past fortnight and is threatening to turn its guns on the opposition at the same time as it tries to engage the Syrian regime.

"For Isis, it is crucial to control such a long stretch of border with Turkey, because it wants to continue the influx of foreign fighters," the western diplomat said. "Aleppo is the key to all of northern Syria. The group has led large numbers of forces from Anbar to Aleppo in preparation for this battle. They are being led by the senior emir, Abu Wahib."

Western leaders have indicated that a key strategy in tackling Isis will involve trying to deprive Isis of the support of the 20 million Sunni Arabs who live between Damascus and Baghdad. But the difficulties of that approach was underscored on Friday when Shia militia shot dead dozens of Sunni in a village north of Baghdad.

Morgue officials said 68 people died in the settlement 40 miles from Baquba, in one of the deadliest such attacks this year. It is the multiplicity of such attacks that has persuaded Sunni Iraqis that they stand a better chance working with Isis than with the Shia-dominated Iraqi state.

"Sectarian militias entered and opened fire at worshippers. Most mosques have no security," lawmaker Nahida al-Dayani told Reuters. "Some of the victims were from one family. Some women who rushed to see the fate of their relatives at the mosque were killed."

A Sunni tribal leader, Salman al-Jibouri, said his community was prepared to respond in kind. "Sunni tribes have been alerted to avenge the killings," he said.