The United Nations Se-curity Council is expected to adopt a binding resolution this week that would require nations to bar their citizens from traveling abroad to join terrorism organizations, part of a U.S.-led effort to galvanize the international community against what Obama administration officials call an "unprecedented" threat from extremists flocking to Syria and Iraq.

Obama administration officials touted the measure, which they said had been negotiated over several months, as a significant step in their strategy against the Islamic State group and other militant organizations that are drawing Europeans, Americans into their violent orbit. But they acknowledged that the UN resolution has no enforcement mechanism and that the international community has no single definition of what constitutes a terrorist group.

"This is really designed to sort of elevate the collective nature of the threat," a senior Obama administration official told a group of reporters Monday, speaking under ground rules that she not be identified.

The U.S. and many European nations already have laws on the books that allow them to prosecute their citizens who attempt to or succeed in traveling to join extremist groups. The UN resolution is intended to prod other countries, such as Saudi Arabia, Qatar and Turkey, to step up efforts to stop the flow of foreign fighters. It is also designed to facilitate more sharing of travel data and other intelligence designed to allow the tracking of foreign fighters, the officials said.

The U.S. has been dealing for more than decade with the problem of Islamic extremists flocking to various battlefields, including Afghanistan, Iraq, Pakistan and Yemen. But the movement of an estimated 15,000 foreign fighters to the civil war in Syria, which has spilled into Iraq, is an "unprecedented flow," that creates an increased risk that some of those people will return to their home countries to attempt terrorist attacks, officials said.

Officials are particularly concerned that a cell of veteran al-Qaida operatives called the Khorasan group is trying to recruit Westerners to attack U.S. aviation with the help of Yemeni bomb makers. And they are also worried about the presence of foreigners within the Islamic State, including the militant with the British accent who appeared to behead two American journalists and a British aid worker.

U.S. intelligence agencies are working to track people traveling to fight with extremists in Syria, but there are major gaps. An Obama administration official said Monday that the U.S. "didn't have full knowledge" of the travel patterns of Mehdi Nemmouche, a Frenchman who returned to Europe this year after fighting in Syria.

On May 24, prosecutors say, he methodically shot four people at the Jewish Museum in central Brussels. Three died instantly, one afterward. Nemmouche was arrested later, apparently by chance.

The U.S. also failed to detect when Moner Mohammad Abusalha, an American who grew up a basketball fan in Vero Beach, Florida, traveled back home from the Syrian battlefield. He later returned to Syria, and in May killed 16 people and himself in a suicide bombing attack against Syrian government forces.

President Barack Obama is expected to lead the UN Security Council session that begins Wednesday, just the second time a U.S. president has done so. Obama chaired a session in 2009 on non-proliferation. It's the sixth time such a session has convened the heads of government, a U.S. official said.