White House officials on Monday acknowledged for the first time that Americans who’ve fought alongside the Islamic State terrorist group in Syria and Iraq have returned to the U.S., raising concerns about an attack on the homeland as President Obama prepares to urge the United Nations to confront the threat overseas.

Counterterrorism experts believe at least 100 Americans have traveled to the Middle East to join the terrorist group, and a senior administration official said the total includes “those who’ve gone, those who’ve tried to go, some who’ve come back and are under active [investigation] — the FBI is looking at them.”

In a little-noticed speech last week, one lawmaker said as many as 40 radicalized Americans have returned to the U.S. after fighting with the Islamic State.

“It is … believed that some 40 of those who left this country to join up with [the Islamic State] have now returned to our country,” said Rep. Timothy H. Bishop, New York Democrat. He said the 40 individuals “are under FBI attention and surveillance. So they are known and being tracked by the FBI.”

The development heightens one of the administration’s gravest concerns about the terrorist group that Mr. Obama once referred to as a “JV” organization: that battle-hardened jihadis holding U.S. passports could slip into the U.S. undetected and plan attacks virtually anywhere.

The revelation also came as Abu Muhammad al-Adnani, a spokesman for the Islamic State, issued a new warning to America. “You will not feel secure even in your bedrooms,” he said.

He called on followers to attack citizens of the U.S., France, Canada and other allies, and ridiculed Secretary of State John F. Kerry as an “uncircumcised old geezer.”

Security analysts say the biggest threat from the Islamic State is those Americans who are able to travel to Syria and Iraq and train directly with the jihadists.

“The threat is primarily from the current U.S. passport holders that have been able to make it out of here,” said Dafna Rand, deputy director of studies at the Center for a New American Security. “The secondary threat is to stop the pipeline of Americans that want to go fight there.”

Ms. Rand pointed to a CNAS report she wrote that found the Islamic State is treating foreign fighters differently than other extremist organizations. Where groups such as al Qaeda would often use foreigners as suicide bombers, the Islamic State is instead viewing them as something much more valuable: citizens who can return to their own nations to start terror cells and plot attacks.

President Obama has said U.S. intelligence agencies have not yet detected any specific plots by the Islamic State to attack targets in the U.S. He will preside at a meeting of the U.N. Security Council on Wednesday to lobby foreign leaders to join a U.S.-led coalition to fight the Islamic State. One measure he’s seeking is tighter visa restrictions to prevent some would-be militants from traveling to Syria and Iraq.

“This is an item of the highest urgency,” a senior administration official said.

But first, the president’s schedule in New York calls for him to give a speech on climate change Tuesday at the U.N. He’s hoping to burnish his environmental legacy by forging a new global climate change pact to be signed in Paris next year.

At the U.N. Security Council on Wednesday, Mr. Obama will become only the second U.S. president to chair such a session.

Mr. Obama is expected to urge the U.N. Security Council to pass a sweeping new resolution that would impose global travel bans on fighters intent on enlisting in overseas wars. Administration officials have said they believe the resolution has enough support to be approved.

Other measures sought by the U.S. will include the U.N. freezing assets of such fighters and Interpol coordinating efforts to monitor the transit of fighters.

The president’s address to the U.N. General Assembly also will focus on his strategy for fighting the Islamic State.

American-born terrorists pose a significant threat, experts say. A major concern for law enforcement personnel has been a repeat of attacks like the Boston bombings in April 2013, which were carried out by a pair of radicalized brothers, killing three people and injuring hundreds more.

Counterterrorism officials also have warned that jihadists could be inspired by other attacks such as the June murder of 19-year-old Brendan Tevlin by Ali Muhammad Brown, whom police now say may be connected to several other killings across the country.

“Their experience and motives are often distinct, but they are increasingly savvy and willing to act alone,” FBI Director James B. Comey told Congress last week. “They may gain inspiration from terrorist narratives, including material in English — events in the United States or abroad perceived as threatening to Muslims … or their own grievances.”

The Justice Department has announced new initiatives for working closely with local law enforcement in identifying and tracking potential extremists in their jurisdictions. The FBI’s psychological analysts have been employed to develop profiles of would-be radicals, trying to get into their heads to counter recruitment tactics and anticipate violent action.

In an address to Americans earlier this month, Mr. Obama discussed the threat posed by the fighters as he announced expanded U.S. airstrikes against the Islamic State in Iraq and in Syria.

“Our intelligence community believes that thousands of foreigners — including Europeans and some Americans — have joined them in Syria and Iraq,” Mr. Obama said. “Trained and battle-hardened, these fighters could try to return to their home countries and carry out deadly attacks.”

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