Islamic State extremists are pushing to secure the border between Turkey and north-western Syria as the main gateway for recruits to join the caliphate they have imposed across much of eastern Syria and western Iraq.

Large numbers of jihadists from Islamic State (formerly Isis) are moving this weekend towards the Turkish border area, about 60 miles north of Aleppo, in columns of armoured trucks that they looted from abandoned Iraqi military bases. The area is now one of the most active front lines in the group’s attempt to redraw the borders of the Levant, a campaign that will have huge ramifications for Turkey.

Residents and Syrian opposition militants in the town of Marea, close to the Turkish border, on Saturday said that Isis had advanced to within sight of the town and had sent envoys to negotiate access.

“They could storm in like the Mongols, if they wanted to,” said a fighter from Syrian rebel group Islamic Front. “But they’re trying to be nice. We have dealt with them before. There is no reconciling with them. We will have to fight.”

The Syrian opposition fought a bitter and costly war with Isis in the same area in January, ousting them from ground they had used as a rallying point for foreign fighters and for a successful push into Iraq. The six-week battle cost the lives of more than 2,500 opposition fighters and allowed the Syrian regime, together with its proxies, to slowly encircle Aleppo from the north-west, a move which is likely to prove decisive in the Syrian civil war.

Since that battle, the flow of foreign fighters from across the Turkish border to Isis has slowed. Isis now wants to reverse that, making it easier for anyone who wants to join them to cross a 130-mile strip of the frontier that has been used by the vast majority of foreign fighters, including British and European jihadists.

“The Turkish border is the only way to smuggle oil, weapons and foreign fighters into [Iraq and Syria],” said Dr Hisham al-Hashimi, an Iraqi expert on Isis. “If it’s closed, it will cut three things: funding, an entrance for the foreign fighters and links to Europe which they are trying to open. If those plans are destroyed, they will aim for another gate to Lebanon.”

Isis’s self-declared new caliph, Abu Bakr al-Baghdadi, has urged bureaucrats, judges, administrators and doctors to relocate to what he claims will be an autonomous area across much of Iraq and Syria that is ruled by hardline Islamic law and pays no heed to existing borders.

European governments and the US have for the last 18 months been urging Turkey, which is a Nato member, to do more to stop jihadists who cross into Syria. Officials in Ankara had at first insisted that there was little that they could do to distinguish between religious pilgrims travelling to Turkey and those who intended to join a jihad.

Intelligence officials insisted that countries concerned that their citizens might be extremists should sound the alarm before they travel. However, European governments have been increasingly frustrated by what they perceive as Turkey’s lack of will to confront the jihadists, given that they were destabilising the Assad regime.

Some agencies believe the Turkish National Police are more willing to interdict Isis than the country’s national intelligence agency, MIT. However, sources have told the Observer that the police have been sidelined in a power struggle with president-elect Recep Tayyip Erdogan, whose circle has given responsibility for jihadists to MIT.

Western officials told the Observer that they were obliged to tread carefully when talking to the Turks about foreign passport holders suspected of trying to travel to Syria through Turkey. Using the term “extremist” or “terrorist” in official correspondence would generally lead nowhere, but Turkish officials were more forthcoming when inquiries were made about “those who abuse religion”.

The battle over semantics underscores the deepening sensitivity surrounding the fast-growing regional extremist threat that some senior figures in the Middle East and Europe say Turkey has facilitated either through neglect or undeclared policy.

“Let’s see how they react to the latest Isis advance,” said one regional leader on Saturday. “For more than a year now people have been telling them this has got out of control. They have to seal their border now. This so-called caliphate cannot be allowed to stand.”

Al-Hashimi said Ankara would now be forced to act. “This time Turkey will do something and block the borders because they don’t trust Isis any more after they attacked Kurdistan. They understand now that Isis could turn on them.”

The stretch of border used by jihadists to infiltrate Syria is a mix of flat plains and rugged ranges, much of it difficult to patrol. Since May 2012, Turkish officials have allowed weapons and supplies destined for recognised Syrian opposition groups to cross. Isis has not relied on a foreign patron to build its capacity, instead looting from armouries, state-owned enterprises and banks. However, it has sold oil from fields that it commands in eastern Syria to Turkish officials, and to regime connections in Damascus.

Isis continues to alarm the region with its capacity to fight concurrent battles on several fronts – a trait on display in the last three days in Syria, where the advance on the north is taking place as the group also tries to seize the regime’s last remaining airbase in eastern Syria.

Regime reinforcements were on Saturday continuing to defend the base, known as Tabqa, where 800 to 1,000 soldiers and airmen, as well as fighter jets, remain stationed. The base’s isolation, however, will make holding it difficult.