As many as 20 to 30 former Guantanamo Bay detainees -- some of whom were released within the last three years -- are suspected by intelligence and Defense officials of having joined forces with the Islamic State and other militant groups inside Syria, Fox News has learned.

The development has cemented fears that the U.S. military would once again encounter militants taken off the battlefield.

The intelligence offers a mixed picture, and officials say the figures are not exact. But they are certain at least some of the released detainees are fighting with the Islamic State, or ISIS, on the ground inside Syria. Others are believed to be supporting Al Qaeda or the affiliated al-Nusra Front in Syria.

A number of former detainees also have chosen to help these groups from outside the country, financing operations and supporting their propaganda campaigns.

Sources who spoke to Fox News were not able to provide the identities of the fighters.

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Senior Defense and intelligence officials say the vast majority of detainees released from Guantanamo don't return to the fight -- and of those who do, relatively few have made it to Syria.

Of the 620 detainees released from Guantanamo Bay, 180 have returned or are suspected to have returned to the battlefield.

Of those 180, sources say 20 to 30 have either joined ISIS or other militants groups in Syria, or are participating with these groups from outside countries. Officials say most of those 20 to 30 are operating inside Syria.

Top military officials on Thursday acknowledged such recidivism but insisted most do not return to the battlefield.

"We know that some of the detainees that have come out of Guantanamo have gone back to the fight, the battlefield. We're aware of that and we think that overall the policy of getting to close Guantanamo is clearly in the interest of the United States, as the president has articulated," Defense Secretary Chuck Hagel said.

Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said: "We believe that the recidivism is a relatively small fraction of those detainees which have been placed into conditions where the risk of recidivism is mitigated. But even one would not make someone wearing the uniform very content."

The development underscores just one of many long-running complications for efforts to shutter Guantanamo Bay, a promise President Obama made within hours of taking the oath of office in 2009.

Nearly six years later, that effort has run aground, complicated by problems with relocating prisoners, by concerns about fighters returning to the battlefield and by Congress' resistance to allowing any to be detained on the U.S. mainland.

Asked if he's concerned about more Guantanamo prisoners being released, former U.S. ambassador to Iraq James Jeffrey told Fox News this has been a concern for a while.

"The majority have remained there, but there was always, if you will, a certain seepage," he said. "These people are ideologically and essentially religiously committed to their evil cause, and it is very hard to sort out who are going to stay at home and who are going to return to the battlefield."

A majority of the jihadists released to their home countries tend to stay and fight locally. Afghans who return to the battlefield, for instance, tend to stay in Afghanistan.

But these officials said the former detainees who have joined ISIS in Syria have migrated from the European and African countries which agreed to receive them from the United States.

Egypt and Tunisia, as well as six European countries, are among them.

According to a source, there are 149 detainees still at Guantanamo Bay, almost 90 of them from Yemen. Eighty detainees currently are eligible for transfer.