The Islamic State remains a global threat despite losing the once vast territory it held in Syria and Iraq, U.S. officials said Thursday in warning about persistent violence from underground cells and an expansion of militants into new areas.

Ambassador James Jeffrey, the State Department envoy to the international coalition fighting the Islamic State, told reporters that thousands of the extremist organization's fighters are scattered around Syria and Iraq, where officials see a "persistent, resilient, rural terrorist level of violence" in that country.

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The U.S.-backed Syrian Democratic Forces ousted Islamic State militants from the last piece of their self-declared calilphate earlier this year. But "the ISIS brand lives on around the world," State Department counterterrorism coordinator Nathan Sales said as he joined Jeffrey to provide an update on the fight against the organization

"ISIS branches and networks now span the African continent from east to west and north to south," Sales said. "They've increased the lethality of their attacks, they've expanded into new areas, and they've repeatedly targeted U.S. interests."

Sales also said the U.S. is also urging countries to take back and prosecute foreign fighters who flocked by the thousands to Iraq and Syria to join the Islamic State.

"Across the coalition, we need to prosecute ISIS leaders, fighters, financiers, and facilitators for the crimes they've committed," Sales said. "That includes building the law enforcement capacity of partner states that have the will to act but might lack the resources or expertise to do so. It also means repatriating and prosecuting foreign terrorist fighters."

President Donald Trump echoed that message outside the White House Thursday, saying, "We have 2,500 ISIS fighters that we want Europe to take...We have thousands of ISIS fighters that we want Europe to take and let's see if they take them. And if they don't take them, we'll probably have to release them to Europe."

The U.S. government has returned two U.S. citizens in recent weeks to face prosecution. The most recent case, announced Thursday in Dallas, involves a 23-year-old man who traveled to Syria to join the Islamic State and was later detained by the Syrian Democratic Forces.