PARIS (AP) — When Islamic State extremists lost control of a key crossroads town in northern Syria in June, some militants shed their jihadi garb and blended in with the flood of Syrians fleeing across the Turkish border. Since then, the exodus of Syrians and Iraqis toward Europe has surged — and Europeans opposed to taking in more refugees say that more than ever, they fear "disguised terrorists" in their midst.

Governments along the route have different assessments of the threat. Two senior Iraqi officials and a Syrian activist say a small group of hardened Islamic State extremists is believed to have left the war zones of Iraq and Syria to blend in with the masses of asylum seekers in recent weeks.

Intelligence officials in France and Germany expressed skepticism, saying they have no specific evidence. The Soufan Group, a security consulting firm, said Monday that some infiltration was probable but the extent of the danger was unknowable, making it "susceptible to exaggeration and exploitation."

The disarray of Europe's asylum procedures in the face of thousands of applications has heightened worries, although security experts say Europe is at far greater risk from homegrown Islamic State sympathizers with valid European travel documents and the means to plan an attack.

Leaders of countries opposed to taking in the refugees routinely cite the fear as one of their primary reasons. The concerns are fanned by lines of exhausted refugees, bedraggled families walking northward along railroad tracks, and trains carrying refugees between countries with minimal or no identity checks, straining a system already near the breaking point.

Even Pope Francis urged caution. In an interview with a Portuguese Catholic radio station broadcast Monday, he recalled that the Bible requires that strangers be welcomed, but acknowledged the need for precautions.

"It's true that 400 kilometers (250 miles) from Sicily there is an exceedingly cruel terrorist guerrilla group, and it's true there's the danger of infiltration," Francis said.

U.S. intelligence officials also are worried that Europe's disorganized response will allow terrorists to circumvent checks.

"We don't obviously put it past the likes of ISIL to infiltrate operatives among these refugees," Director of National Intelligence James Clapper said Sept. 9, using an alternate acronym for the Islamic State group. He said the U.S. is aggressively screening the small number of Syrian refugees it has accepted, but is not so sure every European country is using similar vigilance.

A top Iraqi intelligence official said 20 Islamic State fighters from Syria and Iraq, trained and selected by senior IS operatives, are believed to have slipped into Europe in recent weeks to plan attacks. The chaos of the mass movement northward "is a chance that will never be repeated," he said. The official declined to say where the men were believed to have landed or whether they are coordinating with each other, but said the Iraqi government is working with European authorities to trace them and learn their plans.

Both he and a second Iraqi official said as many as 100 suspected defectors have also made it through, fleeing the grip of the extremist group the only way they safely can. The Iraqis said the defectors were discovered when they failed to show up for morning roll call and have not been tracked beyond Turkey.

A Syrian activist who works against both the regime and Islamic State also said a handful of extremists traveled to Europe. He said there were indications that many more made the journey in the past three months from the northern provinces of Hassakeh, Raqqa and Deir al-Zour, through Turkey and then European shores. The activist said he saw one of the fighters being interviewed on television as a refugee.

The Iraqis and the Syrian spoke on condition of anonymity — the former because they were not authorized to talk to the media about the sensitive information and the latter because he feared for his safety and the lives of colleagues who gathered the information.

Other activists expressed doubt that significant numbers of extremists were among the refugees. A Syrian living in Turkey who goes by the name of Abu al-Hassan Marea said it is not far-fetched that some IS members have infiltrated the refugees in Europe but he knew of no organized initiative. The overwhelming majority of refugees are fleeing the war: "From the tens of thousands, there are maybe one or two," he said.

Marea said he believes the Islamic State group's primary aim is to draw Western militaries into Syria, not to go out to meet them on their home turf.

An edict in Islamic State's Dabiq magazine bolstered that view and offered indirect evidence that defections are a problem.

Deadly attacks this year in France were carried out by homegrown extremists, not migrants from the Mideast. But the Islamic State group's repeated threats to strike in Europe have left many fearful and have fueled the arguments of people opposed to accepting more refugees. It's not clear how imminent the threat to Europe might be compared with the thousands of Syrians and Iraqis fleeing violence in their homelands.

Marc Trevidic, until recently a top French terrorism judge, criticized a group of French mayors who said they would give priority to Christian refugees because of fears of "disguised terrorists."

"Because there is a risk that a few people among the migrants may be from Islamic State, we should do nothing? That's the talk of those who do not want to accept migrants, but it's a little too simplistic," he told BFM television last week. "They have absolutely no need of that to send people. There are enough French, Belgians who have passports or identity cards that can return to carry out attacks."

In addition, there are other extremist groups and no shortage of hard-liners in Syria and Iraq, some of them affiliated with al-Qaida, which has also vowed to carry out attacks in the West.

In June, when the Islamic State group seized the frontier town of Tal Abyad, thousands of Syrians fled into Turkey in chaotic scenes, with some crossing illegally through holes in fences. At the time, activists said the militants were mingling with the refugees and trying to hide their identities.

Magnus Ranstorp, a Swedish counterterrorism expert, emphasized that although it was probable people with links to terrorist groups are among the migrants from Syria and Iraq in the recent waves, Europe already is coping with the risk of its own radicalized citizens. Efficient screening during the asylum process — which includes fingerprinting, photos and a cross-check of European records — can reduce the risk, he said.

"There's no way one can guarantee 100 percent that terrorists are not coming through," Ranstorp said.

Any numbers of extremists are likely to be small, but the issue is highly sensitive in Europe, both on the far right and among mainstream politicians.

"We haven't any terrorism in our region and I don't want to somehow escalate the situation and increase the possible threat level," said Raimonds Vejonis, the president of Latvia, where in August about 300 people protested the government's decision to accept 250 refugees.

"We know that among refugees, there also could be some terrorists, maybe. And that is the main fear of our society," he told AP.

A spokeswoman for the German Interior Ministry said they have "no reliable information" that any individuals or groups have tried to enter Germany amid the migrant flood. Germany's foreign intelligence service, the BND, refused to comment on the specifics offered by the Iraqi officials.

A BND spokeswoman pointed to a Sept. 7 interview by BND head Gerhard Schindler with the newspaper Bild, where he said "it is still unlikely that terrorists would attempt to recklessly cross the Mediterranean in a boat to get to Europe. They could do that with false or stolen papers and a plane ticket much more easily." Neither spokeswoman would be quoted by name.

The top Iraqi official said going into Europe with a false document is more of a risk and can only be done on a very small scale.

German Chancellor Angela Merkel has pointed out that many of the Islamic State's fighters are European.

"The fight against Islamic State reminds us again and again that fighters there come from our countries — from Germany, Great Britain, France, from European countries," she said. "That means we can't just say that there's a problem somewhere out there; it concerns us."

El Deeb reported from Beirut; Abdul-Zahra from Baghdad. Associated Press writers Zeina Karam in Beirut; David Rising in Berlin; Rayyan Sabet-Parry in Riga, Latvia; and Ken Dilanian in Washington also contributed.