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Two separate reports of groups of America-bound Syrians detained below the U.S. southern border and the arrests of six other Middle Eastern men nabbed with smugglers in Arizona in recent days are raising concerns that Islamic State militants could be probing security – and stoking fears some may already be here.

On Monday, five Pakistani nationals and one Afghan were nabbed in Arizona along with two suspected smugglers, a Department of Homeland Security official confirmed. Then, on Tuesday, Honduran authorities arrested five Syrians they said were headed for the U.S. with stolen or doctored Greek passports, but later said the men were college students fleeing war at home. On the same day and 1,800 miles north, two Syrian families were taken into custody at a border checkpoint in Texas.

“Members of two Syrian families, two men, two women and four children, presented themselves at a port of entry in Laredo,” a Department of Homeland Security spokeswoman told FoxNews.com Thursday. “They were taken into custody by CBP and turned over to ICE for further processing.”

"We know that terrorist groups look for the weakest link, or any way they can gain entry."

— Jessica Vaughan, Center for Immigration Studies.

Although sources said it did not appear the Syrians nabbed at the U.S. border were trying to sneak in, word spread among Border Patrol agents, whose union leaders warned them to be extra vigilant.

“Our agents have heard about Syrians being apprehended in the area from other federal agents,” Border Patrol agent and National Border Patrol Council Local 2455 President Hector Garza told Breitbart news service, adding that the union “will be issuing an officer safety bulletin advising Border Patrol agents to exercise extra precautions as they patrol the border.”

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    On Monday, the sx men from Pakistan and Afghanistan were caught after making it across the border.

    “U.S. Border Patrol agents in Sonoita, Ariz., apprehended five Pakistani nationals and one Afghan national Monday," a DHS spokeswoman said, adding that their identities were checked against law enforcement and national security related databases, revealing no "derogatory information."

    All six are in federal custody.”

    The incidents came less than a week after the deadly attacks in Paris and as President Obama and mostly Republican lawmakers engaged in a heated debate over admitting Syrian refugees. Whether sneaking in illegally or entering the U.S. through the refugee program, Middle Eastern immigrants are raising fears that terrorist groups -- including ISIS and Al Qaeda -- could be planting sleeper cells in the U.S. to mount attacks like the coordinated ones in Paris last Friday that left 129 dead and hundreds more injured.

    Two Syrian families "presented themselves" to Border Patrol officials in the Laredo, Texas, sector, according to a DHS spokeswoman. (AP)

    While authorities in both the U.S. and Honduras dismissed any threat posed by the Tuesday incidents, both the U.S. southern and northern borders are ripe for exploitation by terror groups, according to immigration experts.

    "We know that terrorist groups look for the weakest link, or any way they can gain entry," said Jessica Vaughan, director of policy studies for Center for Immigration Studies. "It is likely that terrorists have already managed to get through. The Border Patrol catches only a fraction of the people who try to cross illegally, perhaps 40 or 50 percent. They have already caught a number of aliens from countries associated with terrorism, but we can’t be confident they have caught everyone who has tried.

    "To make matters worse, there is very little interior enforcement, so once they get past the border, they are relatively free to operate here, rent a place to live, get a driver’s license in some states, just like the 9/11 terrorists did," she added.

    On Wednesday, the House Homeland Security committee released a report that found two Al Qaeda terrorists from Iraq who had killed American soldiers were able to enter the country as refugees. The pair, Waad Ramadan Alwan and Mohanad Shareef Hammadi, settled in Bowling Green, Ky., after killing American soldiers. In 2010, they were caught with a machine gun and a missile launcher that they planned to smuggle to insurgents in Iraq.

    “I wouldn’t be surprised if there were many more than that,” said Rep. Michael McCaul, R-Texas, the chairman of the House Committee on Homeland Security. “And these are trained terrorists in the art of bombmaking that are inside the United States; and quite frankly, from a homeland security perspective, that really concerns me.”

    The Honduran arrests came after the suspected Syrians landed at Toncontin airport near Tegucigalpa and failed to make it past airport security checks, a police spokesman said.

    “Five Syrian citizens have been detained and will be taken to our offices to be investigated because it is suspected they are carrying false documents, passports stolen in Greece," Honduran police spokesman Anibal Baca told reporters.

    The men had traveled by air from Syria to Lebanon, then to Turkey, Brazil, Argentina, Costa Rica and on to Honduras. They were planning to make their way over land through Guatemala and Mexico and on to the United States, Baca told AFP.

    "We are not saying they are terrorists," Baca said. "They are being investigated for using false passports. It could be they are fleeing war. That is being investigated."

    In September, a Syrian national reportedly attempted to enter the U.S. illegally through Texas using a passport that belonged to someone else, but authorities did not bring charges.