The Islamic State is setting up networks to support the systematic movement of terrorists from the Middle East to Europe and Asia, according to U.S. defense officials.

"ISIS has several facilitators in place that assist the flow of fighters to Europe," said one official who noted the group is exploiting travel networks used for large-scale human trafficking in the region. The networks include an organizational structure that has been discovered by U.S. and allied intelligence agencies to be using specific groups to help terrorists reach European soil. ISIS has assigned support personnel to the networks to facilitate the process.

No specific numbers were provided by the officials on ISIS fighters moving from strongholds in Syria and Iraq to Europe and Asia.

Many of the fighters, however, are returning nationals who joined ISIS several years ago where they received training and experience during Middle East conflicts. The fighters are regarded as hardened jihadists who will seek to infiltrate society and prepare for future attacks.

A second official said the use of human trafficking networks and refugees by ISIS has increased the danger that similar tactics will be used by the group to send fighters into the United States.

"Refugees from the Middle East could be exploited by ISIS to target the United States," this official said.

The Trump administration is currently battling U.S. courts over President Trump's executive order restricting travelers from entering the United States from six majority Muslim nations.

"There is always a possibility that a foreign fighter from a visa waiver country that has not been detected could return home to fly to the U.S. and conduct an attack," the first official said.

"Our foreign fighter databases are good and information sharing is constantly improving but this scenario is plausible," the official said. "Let's not forget we have plenty of U.S. citizens that went to the so-called caliphate and the caliphate has over 100 nationalities on its bench."

The threat was highlighted in Brussels Tuesday, where military security guards shot and killed a suspected terrorist who set off a small explosive device at a train station.

ISIS fighters also are moving in increasing numbers to Asia but it does not appear that ISIS is shifting its focus to Asia, the officials said.

"ISIS's business model is to set up affiliates around the world they can leverage for worldwide attacks," the first official said. "I think instead of shifting I would say attempting to expand with varying results."

Asked about the overall ISIS threat to the United States and Europe, the officials said vehicle ramming attacks are increasing.

"The new tactic of renting or stealing large vehicles to ram in to crowded areas and ISIS' own endorsement of these tactics present daunting challenges to intelligence and law enforcement officials," the first official said. "We must work together to mitigate evolving threats."

A report by the State Department's Overseas Security Advisory Council noted that in the past six months, Islamic terrorists have conducted seven attacks in Germany, France, Sweden, and the United Kingdom, causing more than 50 deaths.

The danger is increasing, according to the June 7 report.

"The elevated Islamic extremist terrorist threat in Western Europe is expected to persist in 2017," the report said.

"Some authorities assess that as ISIS continues to lose ground in Iraq and Syria, the group will focus on conducting attacks against the West in an attempt to maintain legitimacy among supporters," the report added. "Operations and plotting may increase during times the group may consider symbolic or more permissive, such as holidays and busy travel months."

The report said authorities in Europe have prevented a number of attacks since the beginning of the year. "The frequency and reach of terrorist activity affirms that the threat persists throughout the region," it states.

The Islamic terror group is facing intense pressure from intelligence and law enforcement forces, according to the two officials with access to intelligence reports of the activities.

"The southeastern European region has been the gateway to Europe for foreign fighters wishing to travel west," the first official said, adding that U.S. and allied nations are working hard to disrupt the logistics and support networks.

"This is especially reflected in ISIS' rhetoric and propaganda—they are openly encouraging people to stay home and conduct knife and truck attacks," the official added.

According to Interpol, over 4,000 human traffickers are operating in Europe and human trafficking within the continent is increasing.

Because of the ease of travel in Europe, "ISIS can leverage affordable modes of transportation to transverse Europe if they have documentation that will pass the scrutiny of border guards," the official said.

A lack of coordination among European security services and porous borders that require minimal identification has boosted travel by ISIS fighters.

Army Gen. Curtis Scaparrotti, commander of the European Command, said in a speech last April that the battle against terrorism is difficult.

"This fight against terror and violent extremists will not be easy, nor will it be fast," he said. "It is not a war of choice. It will take resources, determination, and resolve to see the end of terror in Europe."

The European Command said the ISIS infiltration threat is a concern.

"To the south, we see a much more multi-faceted challenge of ungoverned spaces and unresponsive governments resulting in migrant flows of criminality, terrorism and foreign fighters in and out of these areas," the command said in a statement.

"We are putting pressure on ISIL in many avenues," the command said. "This is a long-term effort and not just a short-term challenge that we need to be thinking about. It will take a dedicated effort, not only from the air, but also on the ground to counter transnational threats."

A key focus of the command is assessing threats to American forces and securing service members, civilians, family members, and facilities.

Meanwhile, the Pentagon announced on Tuesday that a key Islamic terror leader, Turki al Bin'ali, was killed in an airstrike in Syria May 31.

"Al Bin’ali had a central role in recruiting foreign terrorist fighters and provoking terrorist attacks around the world," the Pentagon said in a statement.

"As chief cleric to ISIS since 2014, he provided propaganda to incite murder and other atrocities, attempted to legitimize the creation of the ‘caliphate,' and was a close confidant of Abu Bakr al-Baghdadi."

A Bahraini national, Bin'ali, 32, also used his propaganda writings and recorded lectures to attempt to "justify and encouraging the slaughter of innocents."

The terrorist leader was active in recruiting ISIS fighters from Persian Gulf states to join the terror group in Syria. He also was involved in funding operations and giving propaganda lectures in Syria. He also sought to recruit rival al Qaeda terrorist leaders to join ISIS.