LONDON - In the largest antiterrorist sweep in nearly two years, British police on Monday netted a dozen men accused of plotting a large-scale terror attack on targets in the United Kingdom.

The action took place as European concerns over terrorism spiked after a suicide attack in Sweden by a British resident, terrorism arrests in Spain and France, and alarms in Germany over reported threats of a terrorism attack modeled on the onslaught by gunmen in Mumbai.

But security officials said Monday's raid was thought to be linked to UK targets only and not part of a larger threat to other European countries.

Police who swooped in on the men's houses early in the morning were unarmed, suggesting any planned attack was not imminent and the suspects were not believed armed. The men were arrested in London, the Welsh city of Cardiff, and the English cities of Birmingham and Stoke-on-Trent.

The raid was the largest since April 2009, when 12 men were detained over an alleged al-Qaeda bomb plot in the northern city of Manchester.

Counterterrorism officials declined to give more details of the alleged plot, saying only that the men had been under surveillance for several weeks - an indication that the plot could have been in its planning stages. No details were given as to whether explosives or arms were found.

Officers said the men range in age from 17 to 28. Police have up to 28 days to question them before they must be charged or released.

The men are thought to be British nationals with links to Bangladesh and Pakistan, according to a counterterrorism official who spoke on condition of anonymity. Britain is home to large Pakistani and Bangladeshi communities.

In October, the U.S. State Department advised American citizens living or traveling in Europe to be wary amid reports that terrorists were planning attacks on a European city. But a government official on Monday downplayed reports that the latest raids in Britain were part of larger terror concerns across Europe.

"Although serious, we believe this raid may have been a one-off and not necessarily related to larger European terror plot concerns," the official said on condition of anonymity.

Another government official, who also spoke to the AP on condition of anonymity, said that other plots being monitored within Britain had threads to the Europe-wide plot reported in October but that there were no new credible or specific Christmas terror plots.

At the White House, spokesman Robert Gibbs told reporters he did not know of any American connection to Monday's arrests.