AMSTERDAM — The Netherlands raised its alert level for terrorist attacks to "substantial" on Wednesday, citing an increase in the number of Islamist militants traveling from the Netherlands to Syria, as well as a radicalization of Dutch youth.

"The chance of an attack in the Netherlands or against Dutch interests abroad has risen," the National Coordinator for Security and Counterterrorism (NCTV) said in a statement.

"Close to a hundred individuals have recently left the Netherlands for various countries in Africa and the Middle East, especially Syria."

The agency said individuals fighting for radical Islam abroad could return and "inspire others in the Netherlands to follow in their footsteps."

Political changes in the Middle East and North Africa have made space for an expansion of radical Islamic groups that are no longer under the control of security forces, the agency said.

Dutch police and intelligence services have deployed extra personnel to investigate suspect individuals and monitor sources, the agency said.

The Netherlands has not suffered a major terrorist attack, but a radical Dutchman of Moroccan origin murdered the provocative filmmaker Theo van Gogh, who was critical of multiculturalism and of Islam, in Amsterdam in 2004.

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Updated 8:30 a.m. ET: Armed British police carried out early-morning raids Thursday in London, arresting six people in what they called a pre-planned anti-terror operation. 

Just hours later, armed police evacuated around 50 people and closed a major toll road in both directions in Central England after a passenger on the bus was reportedly seen pouring liquid into a box, which then started smoking.  

Police later said the operation involving the bus was "not being treated as a terror-related incident."

The operations came amid heightened security ahead of the London Olympics, which begin on July 27, though officials said the terror arrests were not linked to the upcoming games. 

Police didn't identify the suspects, who range in age from 18 to 30 and included five men and a woman. 

However, Mizanur Rahman, who is involved with a Muslim activist group, said all those arrested were British Muslims. He said the three men taken into custody in Stratford, the neighborhood that contains Olympic Park, were brothers and claimed that one was a former police officer. 

A police press officer declined to comment on the claim. 

Resident John Smallshaw said a raid took place at a residence on Abbey Road, only about a mile from the Olympic Park. 

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Police officers stand next to passengers from a bus after it stopped and was surrounded by armed police on highway near Litchfield in central England on Thursday.

He told The Associated Press he was awoken just after 4 a.m. by "five loud bangs in quick succession" and saw police raiding the home. He said he later witnessed "one young man taken on foot to a waiting ambulance." 

Another man was later taken into an unmarked police car, Smallshaw said, adding that plainclothes officers were still at the premises. 

Searches are being carried out at eight separate addresses in east, west and north London and at one business in east London, police said. Rahman identified one of the addresses as being in Old Street, near London's financial district. 

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Police said all the suspects have been taken to a southeast London police station. 

Bus operation
Live aerial video of the bus operation showed around 50 passengers sitting on the road surrounded by armed police. 

Police declared the area safe about four hours after the incident began, the BBC reported.

"This is not being treated as a terror-related incident," a spokesman for Staffordshire Police said in a statement. 

Earlier, a commuter told the BBC he was four vehicles behind the bus when security officials stopped the traffic. 

"I've never seen so many ambulances turning up, also armed police, helicopter and dozens of police cars," the man, Nick Jones, said. "We were told to stay in our cars, keep windows up and not put air conditioning on."

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Intelligence officials say there has been an expected increase in chatter among extremist groups ahead of the games, but there are still no specific or credible threats targeting the Olympics. The terror level is labeled substantial, a notch below severe. A substantial threat level indicates that an attack is a strong possibility

Msnbc.com and The Associated Press contributed to this report.

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North Korea's military has threatened to reduce South Korea’s conservative government "to ashes" in "three or four minutes" – an escalation of its recent belligerent language.

It vowed Monday to launch unspecified "special actions" of "unprecedented peculiar means," an unusually specific warning.

North Korea regularly criticizes Seoul and just last week renewed its promise to wage a "sacred war," saying South Korean President Lee Myung-bak had insulted the North's April 15 celebrations of the birth centennial of national founder Kim Il Sung.

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Its latest threat follows U.N. condemnation of North Korea's launch of a long-range rocket that exploded shortly after liftoff April 13. Washington, Seoul and others called the launch a cover for testing long-range missile technology. Pyongyang said the launch was meant to put a satellite into orbit.

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The North's special actions "will reduce all the rat-like groups and the bases for provocations to ashes in three or four minutes, (or) in much shorter time, by unprecedented peculiar means and methods of our own style," according to the statement by the special operation action group of the Korean People's Army's Supreme Command.

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Terrorist attacks?
Some South Korean analysts speculated the North's statement was meant to unnerve Seoul; others that the North could be planning terrorist attacks.

It seemed unlikely that North Korea would launch a large-scale military attack against Seoul, which is backed by nearly 30,000 U.S. troops stationed in the South, said Kim Young-soo, a professor at Sogang University in Seoul.

However, Dr. Cheon Seong-whun, of the Korean Institute for National Unification, told NBC News that he "wouldn’t be surprised if the North takes some military actions against the South soon given the concrete words announced by the North today.”   

“I believe the North’s statements have passed the rhetoric stage,” he added.

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The North's latest threat, which was carried by its state media, comes amid rising tensions on the Korean peninsula, with both Koreas recently unveiling new missiles.

The animosity has prompted worries that North Korea may conduct a new nuclear test — something it did after rocket launches in 2006 and 2009. South Korean intelligence officials have said that recent satellite images show North Korea has been digging a new tunnel in what appears to be preparation for a third nuclear test.

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South Korea's Unification Ministry said it was examining North Korea's intentions behind the statement; the Defense Ministry said no special military movement had been observed in the North. Officials spoke on condition of anonymity, citing office rules.

Relations between the Koreas have been abysmal since Lee took office in 2008 with a hard-line policy that ended unconditional aid shipments to the North.

In Beijing, North Korea's biggest ally, China's top foreign policy official met Sunday with a North Korean delegation and expressed confidence in the country's new young leader, Kim Jong Un. 

NBC News' Julie Yoo, msnbc.com staff and The Associated Press contributed to this report.

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