By Jenny Gross and Lisa Fleisher in London and Sam Schechner in Paris 

LONDON--The U.K. government is intensifying efforts to enlist the help of large technology companies such as Twitter Inc. and Facebook Inc. in combating extremist content online amid growing concerns about terrorist threats.

Representatives from the companies, which also include Google Inc. and Microsoft Corp., are due to meet with policy advisers for British Prime Minister David Cameron on Thursday to discuss how they can reduce ways for terrorists to recruit and spread their messages online, according to government officials.

While the large technology companies have been generally cooperative, British officials say, thorny issues remain. Among them: what to do about material authorities consider extremist and want removed but that isn't necessarily illegal, such as some videos of sermons by radical preachers or posts by extremists encouraging Westerners to join the fight in Syria.

Privacy considerations are another challenge in instances where U.K. authorities have asked technology companies to hand over details of the people posting the content, such as names, usernames, email addresses and Internet protocol addresses, which can help identify a person's general location.

Thursday's meeting, which is due to take place at Mr. Cameron's official residence at Downing Street, will be chaired by Jo Johnson, head of the prime minister's policy unit. A spokeswoman for Mr. Cameron said the purpose of the meeting is to discuss "what we can do collectively in this area." She added that the big technology companies have been collaborative in working with the government to remove terrorist and extremist material, though they have raised some concerns in general about data protection.

Facebook, Google, Twitter and Microsoft declined to comment on the Downing Street meeting. Big technology companies say they are generally responsive to government requests in removing terrorist-related content and many have policies against posting violent or threatening content.

Technology companies have tried to push back against some of the requests to remove content or turn over user data, though. Internally, some technology executives say they are worried that censorship techniques more common in countries such as Russia and Turkey could become more generalized as governments grant themselves more power.

Some companies are skeptical about handing information about their users to governments, particularly if the user hasn't done anything illegal, said Michael Clarke, director at the Royal United Services Institute, an independent think tank on defense and security. "It's a very delicate relationship at the moment," he said.

Still, the companies say they seek to work with government. In the U.K., for instance, Google handed over information in response to 1,100 of the more than 1,500 requests it received from the government, according to data released by the company. French police officials say many companies have dedicated pages that allow law enforcement organizations to send requests directly to the firms for information such as names, email addresses, credit card billing information and other information.

Sophisticated and prolific use of social media for propaganda purposes has been a hallmark of Islamic State, the militant group that has captured large stretches of territory in northern Iraq and Syria. Extremists have posted content ranging from images of killings to promotional-type videos intended to lure young Westerners to fight.

The concern for many European countries, including the U.K., France and Belgium, is that the material will serve to fuel the already large numbers of citizens going to fight with extremist groups overseas--and that they will be more likely to take part in terrorist activity when they return.

U.K. authorities say, on average, five people a week travel from Britain to Syria and Iraq to fight and there has been a sharp increase in the terror-related arrests at home. On Wednesday, police arrested a man and a woman separately on suspicion of terrorist activity as part of separate Syria-related investigations.

As a result, the U.K. and other governments are stepping up efforts to delete content and track down the authors of extremist content online. London's Metropolitan Police, known as Scotland Yard, says it has been removing around 1,000 pieces of such content from the Internet each week, most of which is related to Iraq and Syria. This includes videos of beheadings and other killings, torture and suicides.

"Dealing with material which may be described as extremist, but does not obviously infringe (upon) U.K. terrorist legislation, is more difficult, " a senior U.K. government security official said. "We have proposed to companies that they consider seriously whether this material is consistent with their terms and conditions."

European Union officials met in Luxembourg earlier this month with representatives ofGoogle, Facebook, Twitter and other companies to discuss ways to combat online propaganda from terrorist groups. France has recently beefed up its antiterrorism laws to allow, among other actions, authorities to cut off Internet access for people defending terrorism and websites labeled "terrorist." The measures also permit wider terrorist surveillance online.

But some specialists in counterterrorism question the effectiveness of governments increasing reliance on censorship and filtering to counter online extremism. Ghaffar Hussain, managing director of London think tank Quilliam Foundation, said such moves tend to be costly and potentially counterproductive. He said a more effective method is producing content for online initiatives that counter extremist ideas, such as parody videos making fun of recruits militant group Islamic State.

"To simply shut the debate down doesn't allow any progress to be made on the counter-extremist front," Mr. Hussain said.

Write to Jenny Gross at jenny.gross@wsj.com, Lisa Fleisher at lisa.fleisher@wsj.com and Sam Schechner at sam.schechner@wsj.com

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