Syria crisis has become a matter of U.S. homeland security over concerns about a small number of Americans who have gone to fight with Syrian militants and returned home, new Homeland Security Secretary Jeh Johnson said Friday.

Syria crisis has become a matter of U.S. homeland security over concerns about a small number of Americans who have gone to fight with Syrian militants and returned home, new Homeland Security Secretary Jeh Johnson said Friday.

Johnson said he and other law enforcement and security officials around the world were focused on foreign fighters heading to the bloody war, including those from the United States, Canada and Europe.

In his first major speech since taking office in December, Johnson did not discuss how many U.S. fighters may be in Syria.

"We need to do our best to pay close attention to an evolving situation," Johnson said.

Two U.S. officials said at least 50 Americans have gone Syria to fight. The officials spoke on condition of anonymity because they were not authorized to discuss intelligence publicly.

The U.S. fighters in Syria are recruited by extremists, indoctrinated and provided terror training, according to one of the officials briefed on the threat. More Americans are considering going over, the official said.

Some terror training camps in Syria are filled with westerners, the official said. Some of the Americans who have gone over there to train are already back in the U.S., the official said, citing ongoing investigations around the country. The Americans going to Syria are not all of Syrian decent, the official said, and are from a cross-section of backgrounds from across the U.S.

The State Department has no estimates of how many Americans have gone to fight with Syrian militants, but British defense consultant IHS Jane's puts it at a few dozen. An estimated 1,200 to 1,700 Europeans are among militant forces in Syria, according to government estimates.

To Johnson, it's not just people joining the fight in Syria that are a concern.

"At the same time, extremists are actively trying to recruit Westerners, indoctrinate them and see them return to their home countries with an extremist mission," Johnson said. "Syria has become a matter of homeland security. DHS, the FBI and the intelligence community will continue to work closely to identify those foreign fighters that represent a threat to the homeland."

Johnson also said he is most concerned about "lone wolf" terrorists who haven't received any specific training from al-Qaida or other terror groups but instead have become self-radicalized. He cited those accused in the Boston Marathon bombings as examples of people involved in a particularly worrisome type of terrorism.

"It may be the hardest to detect, involves independent actors living within our midst, with easy access to things that, in the wrong hands, become tools for mass violence," he said.

Johnson said that while it's critical that the U.S. play close attention to communities and report suspicious activity, the government needs to strike a balance on domestic security.

"We have to be sensitive ... with our actions, not breeding those who want to do harm against Americans faster than we can take them out with our actions, not breeding those who want to do harm against Americans faster than we can take them out," Johnson said.

Johnson also touched on immigration in his remarks, repeating earlier statements that immigration reform also is a matter of homeland security. He added that he is confident that immigration reform will be passed but noted that it's up to Congress as to when that happens.