This story has been updated.
Pakistani intelligence officials say that a drone strike inside Pakistan has killed eight German nationals in the Waziristan region of Pakistan. US officials could not immediately confirm the report.
The strike comes a day after the State Department issued a travel advisory for Americans traveling to Europe because of the potential threat of Mumbai-style commando attacks on civilians, possibly by terrorists of German origin based in Waziristan. Authorities learned of the possible plot this summer from a German national who had been training for jihad and is being held in Afghanistan.
In an interview Sunday, Pakistan's Ambassador Husain Haqqani told ABC News that the plot's leaders had been identified and targeted.
"I think that several people who were involved in the plotting have been targeted, and the others are certainly on the radar of U.S., Pakistani and European intelligence services," Haqqani said.
The suspected German militants killed Monday in Waziristan reportedly belonged to a group called Jehad al Islami.
Among the possible targets in the suspected European terror plot are pre-security areas in at least five major European airports, a law enforcement official told ABC News. Authorities believe terror teams are preparing to mount a commando like attack featuring small units and small firearms modeled after the Mumbai attack two years ago.
The State Department issued a highly unusual "Travel Alert" Sunday for "potential terrorist attacks in Europe," saying U.S. citizens are "reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure."
One scenario authorities fear is a repeat of the 1985 attack on the Rome and Vienna airports, when Palestinian extremists threw grenades and opened fire on travelers waiting at ticket counters injuring 140 and killing 19, including a small child.
A curfew was ordered this weekend at Ramstein U.S. Air Force Base in Germany, with soldiers told to remain at home and not to wear uniforms off base "in response to a threat condition," a Ramstein spokesperson said. The U.S. European Command (EUCOM) commented, "With the increased reports of terror activity throughout Europe, the US and its allies are working to ensure the safety and security throughout the region. The US and partner nations are working to investigate and prevent possible terror threats."
But despite public alerts, there are still concerns the terror group may go ahead with attacks.
"If they believe they can pull it off, I think there's every reason to believe they may go ahead and try it," said former FBI agent Brad Garrett, now an ABC News consultant. "These plans go together at a particular point in time, and it may be perhaps that they are at a juncture with people, equipment and locations that they need to do it now."
U.S. Homeland Security Issues Terror Bulletin
ABC News reporters checking European airports over the weekend reported no obvious signs of heightened security, but authorities said such steps would not necessarily be visible.
The U.K. upgraded its terrorism warning to "high" from "general" for France and Germany but would not comment on specifics.
The Department of Homeland Security and the FBI sent a bulletin to local U.S. law enforcement agencies Sunday morning, saying there was currently no information about specific threats to U.S. targets, but they are monitoring recent terrorist threat reports regarding Europe and would continue to "assess."
The bulletin noted al Qaeda affiliates and those who "follow their ideology" may inspire those with "the ability to access the U.S. legally" and expressed concerns about possible attacks using "small arms, lone shooters and small unit tactics."
Federal authorities urged localities to raise general security awareness, including training private security staff to take note of individuals "loitering for no apparent reason, sketching or pace counting."
In a phone conference with reporters Sunday, State Department officials took pains not to discourage travel to Europe with the alert and gave few concrete steps to take.
"Use common sense," said State Department Undersecretary Patrick Kennedy. "If [tourists] see unattended packages or hear loud noises, quickly move away from them."
Escalating Chatter in Jihadi Cyberspace
Authorities have detected a dramatic increase in online chatter among jihadist websites the last week, in what experts believe could be other terrorists banning together in anticipation of terror attack plans in Europe and hoping to engage themselves in prospective plots.
The escalating discussions in the virtual meeting rooms for al Qaeda supporters have praised terror attacks plan and suggested targets, communicating with fellow believers just as the terrorist teams at the center of the current suspected plots likely did, experts said.
"When you've got this set of threats, what you're really seeing is the culmination of online chatter a couple years ago," said jihadi website monitor Aaron Weisburd, an instructor at West Point's Combating Terrorism Center. "These are the guys who represent a real threat right now."
There's real significance to chatter when combined with other elements of intelligence, said Garrett.
"The government's already saying publicly that they are concerned with this level of chatter and they're concerned about U.S. interests in Europe," Garrett said. "I think that makes it particularly important."