WASHINGTON (AFP) - The Syria conflict has become a US domestic security concern, Homeland Security chief Jeh Johnson said Friday, one day after returning from talks in Europe where the war-torn nation topped the agenda.

Johnson returned late Thursday from a meeting in Poland with Interior Ministers from the United Kingdom, France, Germany, Spain, Italy and Poland. He was accompanied on the visit by US Attorney General Eric Holder.

"Syria was the number one topic of conversation for them and for us," Johnson said in his first major speech since assuming his post in late December.

"Syria has become a matter of homeland security," he added, in remarks at the Wilson Center think tank in Washington.

Johnson said that he, as well as security officials in Europe, are paying particular attention to extremists in their respective countries who are traveling to Syria to take up arms.

"Based on our work and the work of our international partners, we know individuals from the US, Canada and Europe are traveling to Syria to fight in the conflict," he said of the nearly three-year-old conflict, which is estimated to have killed more than 136,000 people.

"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," said Johnson.

He added that US intelligence and security agencies "will continue to work closely to identify those foreign fighters that represent a threat to the homeland.

"It's not just in this country. Our European allies are very concerned about this issue," Johnson said.

"Collectively, we're determined to do something about it."