The attack hit a house in North Waziristan. That region has been named as the source of the European terror plot that has prompted American authorities to issue a travel advisory. One or more German citizens are reported to be linked to the plot.
Two officials said the victims were believed to be German citizens in the region for terrorist training. A third said they were believed to be foreigners, but gave no details.
The officials spoke anonymously because their agency does not permit operatives to be named in the media.
The travel advisories from Tokyo and Stockholm came as European authorities sought to calibrate their messages on counterterrorism efforts, hoping to raise public awareness about the threat but without sowing panic.
The warnings could plant the seed for possible damage to Europe's lucrative tourism business at a time when the continent's economy has been coping with recession - though many tourists took the warnings in stride.
The U.S. State Department alert Sunday advised the hundreds of thousands of American citizens living or traveling in Europe to take more precautions about their personal security. The Japanese alert was similar.
Britain's Foreign Office warned travelers to France and Germany that the terror threat there was high. Sweden's Foreign Ministry did not single out any particular countries in its message.
Amid increased security in Paris, 61 soldiers from an Alpine regiment were deployed over the weekend at two sites in Paris, Notre Dame Cathedral and the Sacre-Coeur Basilica, the joint staff of the Paris defense zone said.
One trigger for the heightened concern came from French authorities last month. A parade of officials said France was facing its highest terror alert level in years, pointing to increased violence and threats by al-Qaida's North African branch.
The public concerns intensified last week after a Pakistani intelligence official said eight Germans and two British brothers were at the heart of an al-Qaida-linked terror plot against European cities.
Security officials say terrorists may be plotting attacks in Europe with assault weapons on public places, similar to the deadly 2008 shooting spree in Mumbai, India. European officials have provided no details about specific targets.
ABC News reported that opne 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.
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Authorities in Europe sought to play down any talk of a rift in approach.
"The Americans have got to make their own announcements, that's a matter for them. We work very closely with them on these issues, and have the same policy - which is to update travelers and give them advice as and when we think it's appropriate," Steve Field, spokesman for Prime Minister David Cameron, told reporters in London.
German Interior Minister Thomas de Maiziere said his country has no concrete evidence of an imminent attack and that security forces are vigilant due to an ongoing "high abstract danger" of the terror threat.
Speaking in Berlin, de Maiziere insisted that German authorities have been aware since early 2009 of possible targets in the capital mentioned in U.S. media reports over the weekend.
"There is no reason to be alarmist at this time," de Maiziere said.
Eight Germans and two British brothers are at the heart of an al-Qaida-linked terror plot against European cities, but the plan is still in its early stages, with the suspects calling acquaintances in Europe to plan logistics, a Pakistani intelligence official said last week. One of the Britons died in a recent CIA missile strike, he said.
The Pakistani official said the suspects are hiding in North Waziristan, a Pakistani tribal region where militancy is rife and where the U.S. has focused many of its drone-fired missile strikes.
Britain, France and Germany have not raised their terror threat levels in recent weeks - in part because they are already high.
The Swedish intelligence agency SAPO on Friday raised the terror alert from low to elevated, noting a shift in activities among Sweden-based groups that could be plotting attacks against the country.
In Rome, speaking on state-run RAI TV, Italian Foreign Minister Franco Frattini said the U.S. alarm about the potential for a terror attack in Europe was "realistic" for Italy because it has troops in Afghanistan. Frattini said there were no specific Italian targets.
In Washington, the FBI and the U.S. Homeland Security Department said they have no indication that terrorists are targeting the United States or its citizens as part of a new threat against Europe.
An intelligence bulletin obtained Monday by The Associated Press said the U.S. government organizations said al-Qaida continues to want to attack the United States, but that there was nothing specific, imminent or related to the European plots.
Former U.S. Homeland Security Secretary Michael Chertoff urged Americans in Europe to take commonsense precautions.
"Don't walk around with the American flag on your back," Chertoff told ABC's "Good Morning America." "(Consider) where would you take shelter if something happened."
In New York City, police officers took up positions outside the city's largest hotels today.
Although there is no specific threat here, there is cause for concern.
NYPD Commissioner Ray Kelly said department analysts have studied the Mumbai attacks and that 250 officers have been specifically trained to deal with similar attacks. The NYPD routinely conducts hotel-specific training exercises, he said.
Nick Casale is a former chief of counterterrorism for the MTA.
"I think this one is being handled right. Just because they are saying the potential is Europe doesn't mean that a high profile, high value target like New York should not be vigilant," he said.
Tourists say the increased security is both comforting and unnerving.
"It is unfortunate, but it is the world in which we live and have been living for the last decade or so," hotel guest Lauren Rota said.
In Britain, officials noted that the United States has taken a far more cautious line about terrorism.
British intelligence prefers to keep targets under surveillance as they plan attacks, often waiting until the final stages of a plot to intervene - hoping to gather evidence to be used in prosecutions, and to gain as much information as possible about plotters' contacts.
"That cuts significantly too close to the bone for the United States, they are not happy to let plots run for too long. The U.K. will allow a plot to run to quite a late stage," said Tobias Feakin, director of national security at London's Royal United Services Institute, a military think tank.
Business travelers and tourists arriving Monday at Paris' Charles de Gaulle airport from the United States said they were aware of the warnings but weren't changing their plans.
"I'm very happy to be here in France. I think we're very safe, and I trust the French government to keep us safe," said James O'Connell, a 59-year-old from Pittsburgh, arriving in Paris for a 7-day vacation.
Germans - authorities and citizens alike - were not convinced of the need for concern.
Marian Sutholt, 25, of Berlin said: "If you worry all the time, you actually live up exactly to what the terrorists want."
In Paris, the New York Knicks practiced as scheduled Monday ahead of the next match of their European preseason tour.
"We know where to go and the places to visit and again, you have to cherish the moment because it's not often you get a chance to play an NBA game in Paris," star forward Amare Stoudemire said.
Information from ABC News and The Associated Press included in this report.
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