|Intelligence sources say an al-Qaeda plot for simultaneous attacks in cities across Europe have been foiled [EPA]
A White House spokesman says that Barack Obama, the US president, has been following the security threat in Europe and that in issuing its warning to Americans travelling to Europe, the state department has been "responsive to the president's direction that we spare no effort".
"From the day we became aware of this latest plot, the president made clear we need to do everything possible to disrupt this plot and protect the American people," Nick Shapiro, the White House assistant press secretary, told Al Jazeera.
The state department on Sunday said that public transportation systems and other tourism-related facilities in the continent could be targeted for attacks by al-Qaeda in Britain, France and Germany.
"US citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when travelling," warned a statement from the state department.
The alert also said attackers may use "a variety of means and weapons and target both official and private interests," and that particular targets could be railways, subways and other tourist infrastructure.
The statement said "current information suggests" al-Qaeda and affiliated groups were continuing to plan attacks there.
Al Jazeera's Paul Brennan, reporting from London, said: "The US state department has been aware of the growing concerns of the intelligence that has been growing pace over the past couple of days about the prospect of terrorist attacks.
"What is interesting is the backdrop of this statement. It is clear that European intelligence agencies are still very concerned about details that have been coming out from Bagram air base [in Afghanistan], where it is known they have been interrogating who they call a high value suspect: A man called Ahmed Siddiqui.
"He is a German man of Pakistani origin who is said to have had details of plans by al-Qaeda to launch Mumbai style attacks. That is small cells of highly mobile and armed people in European cities."
Meanwhile, police in Italy have arrested a man of Algerian origin and wanted in France, who was picked up close to the central train station in Naples, allegedly with a bomb-making kit, the Italian ANSA news agency reported.
The 28-year-old man, for whom a Europe-wide warrant had been issued, was arrested on Saturday and was to be extradited to France, the agency said. The report did not give details of the charges.
Italian police had trailed and filmed the man for three days until they moved in on him. They said he had arrived in the southern Italian city in early September and visited two local mosques.
Investigators also seized a computer and a mobile phone, and the counter-terrorism prosecuting authority has opened an investigation, the report said.
Last week, British and US media reported that Western spy agencies had foiled an advanced al-Qaeda plan to attack European cities.
The British Foreign Office has not changed its travel advice, but recommends "vigilance" when travelling to France and Germany.
Germany said it knew of information pointing to possible al-Qaeda attacks in Europe and the United States.
The plot involving al-Qaeda and allied groups would have involved groups of assailants taking and killing hostages, possibly along the lines of the 2008 raid in the Indian city of Mumbai in which 166 people died, the sources said.
Intelligence sources have claimed that an increase in strikes by unmanned US drone aircraft on suspected fighters in Pakistan in the past few weeks was part of efforts to thwart the suspected plot.
Frequent attacks by US drone aircraft have been heavily criticised by Pakistani leaders for killing innocent bystanders and infringing upon national sovereignty.
Pakistan publicly condemns the attacks as violations of its sovereignty, and the attacks are unpopular among the Pakistani people.
But the Pakistani government is believed to assist in at least some of the missile attacks, although the US does not publicly acknowledge the existence of the covert, CIA-run operation.
The UN has called for the US to end the CIA's control over drone attacks, with Philip Alston, the special rapporteur on extra-judicial executions, saying that the US spy agency does not provide enough transparency or accountability for the programme.
Alston said that the use of drone attacks risked creating a "Playstation" mentality towards killing, "because operators are based thousands of miles away from the battlefield, and undertake operations entirely through computer screens and remote audio-feed".