As President Barack Obama prepared to explain his reasons for taking military action against the terrorist group known as the Islamic State, top U.S. intelligence officials told Congress on Wednesday that the organization does not pose an immediate threat to the country.

The Department of Homeland Security is "unaware of any specific credible threat to the U.S. homeland" from the Islamic State, said Francis X. Taylor, the undersecretary for intelligence and analysis.

However, Taylor cautioned that the group "constitutes an active and serious threat within the region and could attempt attacks on U.S. targets overseas with little or no warning."

Taylor's comments to the Senate Homeland Security Committee follows growing concern that the Islamic State could one day attack the United States if it consolidates its gains in Syria and Iraq.

In January, the group's leader publicly threatened a "direct confrontation" with the United States, and militants later beheaded two American journalists after the United States began bombing Islamic State forces in Iraq.

Of primary concern to U.S. intelligence and its allies are the number of foreign fighters from Western countries who have flowed into Syria. Intelligence officials are fearful they could return to their native countries and launch attacks.

In May, a French national who fought with the Islamic State killed four people at a Jewish museum in Belgium. French authorities arrested a man with ties to the militant group earlier this year after he was found with explosive devices.

In his written comments to the committee, Nicholas Rasmussen, the deputy director of the National Counterterrorism Center, estimated that the number of foreign fighters from Europe to be about 2,000. The bulk of them, he wrote, come from Britain, France and Germany.

Rasmussen said more than 100 U.S. persons had traveled to Syria or attempted to go there. He said the figure included those who had been killed in fighting and those who have returned to the United States.

According to a U.S. counterterrorism official, the FBI is monitoring about a dozen people who went to Syria and came back to the United States. It is unclear what they did in Syria.

A handful of Americans are believed to have died in Syria, including a young man from Florida named Moner Mohammad Abusalha, who blew himself up in May while fighting for Jabhat al-Nusra, an affiliate of al-Qaeida.

Abusalha had gone to Syria, where he trained, and then returned to Florida without the FBI's knowledge before heading back to the Middle East.

The FBI has arrested more than a half-dozen people trying to travel to Syria to support the Islamic State.

On Wednesday, a 19-year-old Colorado woman pleaded guilty to one count of conspiracy to provide material support to a designated foreign terrorist organization. The Justice Department said Shannon Conley became engaged to a member of the Islamic State whom she had met on the Internet. She then trained in the United States to fight in Syria. Conley was arrested in April in Denver as she tried to board a flight to Turkey.

About a dozen people from the United States are believed to have joined the Islamic State.

In comments made last week at the Brookings Institution, Director Matthew Olsen of the National Counterterrorism Center said the United States is also concerned that an Islamic State sympathizer -- "perhaps motivated by online propaganda -- could conduct a limited, self-directed attack here at home with no warning."

But he added, "In our view, any threat to the U.S. homeland from these types of extremists is likely to be limited in scope and scale."