ANTAKYA, Turkey — An ISIS operative traveled across the Syrian border late last year, settled in a Turkish port city, and began work on a mission to sneak jihadis into Europe. It has been successful, he said, in an interview near the Turkey-Syria border: "Just wait."

The operative, a Syrian in his thirties with a close-cropped black beard, said ISIS is sending covert fighters to Europe — as did two smugglers who said they have helped. He smuggles them from Turkey in small groups, he said, hidden in cargo ships filled with hundreds of refugees. He said the fighters intend to fulfill ISIS's threat to stage attacks in the West. He views this as retaliation for U.S.-led airstrikes against the group that began in Iraq last summer and Syria last fall. "If someone attacks me," he said, speaking with BuzzFeed News on condition of anonymity, "then for sure I will attack them back."

Western governments worried even before the airstrikes that ISIS would find ways to get its battle-hardened fighters across their borders. The operative is the first ISIS member involved in these plans to discuss them with the press. He detailed a scheme that takes advantage of the worst humanitarian crisis in a generation, which has sent 3.8 million refugees fleeing Syria's civil war, pouring more than 1.5 million into Turkey alone.

From Turkish port cities like Izmir and Mersin, many thousands of these refugees have ventured across the sea, aiming mainly for Italy. Then they often make for more welcoming countries like Sweden and Germany, turning themselves over to authorities and appealing for asylum. The operative said he worked with smugglers to slip fighters into this chaotic human tide. "They are going like refugees," he said.

Two refugee-smugglers in Turkey said they helped ISIS send fighters to Europe in this way. One put more than 10 of them on his ships, then got cold feet when asked to send more, he told BuzzFeed News last fall. Another said he'd been sending ISIS fighters for months and continues to do so. "I'm sending some fighters who want to go and visit their families," he said in an interview in southern Turkey, speaking on condition of anonymity. "Others just go to Europe to be ready."

Among his colleagues in the port city where he works, the smuggler has a reputation for transporting fighters. BuzzFeed News contacted him after meeting the ISIS operative, seeking to investigate the operative's claims. At first he denied that he smuggled fighters at all; then he said he needed "permission" to discuss the issue with a journalist. Soon after he revealed that he worked for the ISIS operative, who confirmed this and allowed the interview.

The smuggler said some fighters were Syrian. He could tell from their accents that others hailed from elsewhere in the Middle East, while still others spoke Arabic poorly, or not at all. Some told him they were from European countries, he said, and a few claimed to be from the U.S.

Despite crackdowns from Turkish authorities, ISIS continues to move its fighters across a porous, 565-mile border that has long been a transit point for jihadis traveling to and from Syria's war. Those who need them are given fake Syrian passports, the smuggler said, which can be relatively easy to obtain. After he receives the fighters, the smuggler said, he puts them up in a hotel, waiting for the passenger list for the refugee ship to fill and the weather to be right. They leave Turkey like any refugees: on small boats that steal them away to cargo ships anchored in international waters. The smuggler said he had 10 fighters waiting in one port city, "and we will send them on the next ship."

The ISIS operative said this method of moving fighters was important to the group because Western governments, along with Turkish authorities, have stepped up efforts to track jihadis returning from Syria, which makes plane travel from Turkey risky. The scrutiny promises to increase as Western capitals work to prevent terrorist attacks like those that struck France this month, leaving 17 dead. ISIS has more than 20,000 foreigners in its ranks, according to one recent estimate, with more than one-fifth of them citizens or residents of Western European countries. If these jihadis return to Europe in refugee ships, they can travel home via open land borders that receive far less scrutiny than airport security. The ships could also land Syrian or other Middle Eastern fighters in Europe amid the confusion of a refugee crisis that worsens by the day.

Two senior members of the U.S. Senate told BuzzFeed News they had knowledge that ISIS is smuggling fighters to Europe on refugee ships — the first confirmation from U.S. Congressmen that this scenario is underway. "I think it's safe to say that that goes on. To what degree, I'm not sure we know," Senate Intelligence Chairman Richard Burr said in an interview on Tuesday.

"We've heard that," said Sen. Lindsey Graham, a member of the Senate Armed Services Committee, adding that as the war in Syria grinds on, refugees and ISIS fighters alike will continue to use Turkey to transit in and out of the region.

"If you don't stabilize Syria, you'll never get a handle on this," Graham said. "You're going to have this dilemma of how do you tell a legitimate refugee versus a jihadist going someplace else."

The ISIS operative, a former member of the Syrian security forces, joined the opposition early in the civil war and led a rebel battalion under the banner of the U.S.-backed Free Syrian Army (FSA). He pledged himself and his men to ISIS a year ago, compelled by its vision of building a hardline caliphate. He continued to work as a commander, battling both rebels and the regime, before moving to a role he described as "security," which involved assassinations. In Turkey, he said, he monitors rival rebel groups along the border in addition to his work sending fighters overseas. "It's our dream that there should be a caliphate not only in Syria but in all the world, and we will have it soon, God willing," he said.

He is a wanted man among his former rebel allies. One FSA commander said he had been hunting for the operative, echoing the details of his defection to ISIS and providing photos of the operative to confirm his identity. "He has killed a lot of FSA leaders," the commander said.

The operative agreed to be interviewed at the urging of a former rebel who fought alongside him early in the war. Sitting at a restaurant in southern Turkey, he had two associates monitor the entrance from a car outside and received calls on his cell phone at 30-minute intervals to confirm that he was safe. He said he had received permission for the meeting from his superior in ISIS — an official referred to internally as an "emir." "There are some things I'm allowed to tell you and some things I'm not," the operative said.

Reached later by an intermediary with established connections to ISIS officials, the emir declined to speak with BuzzFeed News but confirmed that he had approved the meeting with the operative.

At the restaurant, the operative claimed that ISIS had sent some 4,000 fighters to Europe. Given international efforts to clamp down on the group, the number seemed improbably high, and he may have cited it as an attempt to boost the group's stature and spread fear. His other comments suggested a more modest effort in which ISIS struggled to keep ahead of Turkish and Western authorities. "We need to smuggle them quickly," he said of the fighters. But smugglers insisted on waiting until their ships filled well past capacity, the operative said, sometimes with as many as 700 people. "We can't pay for all the refugees just to have enough to send the ship," he said.

The operative said he was forced to move his work from city to city in response to Turkish efforts to crack down on refugee-smuggling. And he worried about being caught. He said he had been advised by his emir to pose as a refugee while in Turkey and to consider finding a regular job as a front.

He said several times that "the whole world" is fighting ISIS, in reference to the international coalition involved in the airstrikes, and said he hoped ISIS attacks in the West would break the coalition's resolve, getting the strikes to stop. He also said he believed any attacks would target Western governments, not civilians — though even if he was sincere, it's something over which he would have no control.

An official with the Turkish foreign ministry said in an emailed statement that authorities were working to combat refugee-smuggling generally. He pointed out that Europe accepts relatively few refugees through legal channels — a fact that likely increases the demand for smuggling. "Illegal migration has been an important issue and Turkey is effectively fighting against it," said the official, who declined to be named. "Of course the most effective way to put an end to all these problems would be immediate action by the international community to solve the Syrian conflict."

The official said Turkey's government was unaware of ISIS smuggling fighters to Europe in refugee ships. "We do not have that particular intelligence," he said.

He added: "Turkey has been taking very tight measures against [ISIS] with all the capabilities the government has."

A prominent refugee-smuggler based in the coastal city of Antalya, who said he did not deal with jihadis, told BuzzFeed News that Turkish authorities had recently questioned him on the subject.

The same smuggler who said he works with ISIS fighters now said authorities grilled him about it too. "I told them no," he said. "All the intelligence agencies around the world are following us and trying to catch us. But if someone asks me if I send fighters, I will say no, I only send refugees to help them find a better life."

With additional reporting by John Stanton in Washington, D.C.