Swedish security services are on high alert in connection with a potential terror threat, officials said. Reports in Swedish media say several Islamic State fighters have arrived in Sweden to carry out attacks on civilian targets in the capital Stockholm.

“Right now we're gathering information and intelligence and coordinating with our national and international partners,” security police (Säpo) press spokesman Simon Bynert told the TT news agency.

He added that Säpo is also cooperating with the national police service.

'Secret cell': At least 400 ISIS fighters trained for attacks on Europe – report https://t.co/ySjU130MEmpic.twitter.com/tv5AQC0lgl — RT (@RT_com) March 24, 2016

“That means we're sharing this information with them to see if they can implement measures that fall under their remit.”

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The Swedish Expressen newspaper reported that the country's security service received information from their Iraqi colleagues about an imminent terror threat in the country.

About seven to eight Islamic State terrorists arrived in Sweden to carry out attacks in Stockholm, Expressen sources claimed.

Speculation has appeared in local media claiming that Swedish King Carl XVI Gustaf's 70th birthday on April 30 could be a possible target for extremists, AP reported. Such events usually bring together Sweden’s royal family, government officials and EU royal visitors.

At least 300 Swedish nationals have traveled to Iraq and Syria to join Islamic State (IS, formerly ISIS/ISIL) according to research conducted by the International Centre for Counter-Terrorism (ICCT), published on April 1.

In March, Europol said that between 3,000 and 5,000 so-called ‘foreign fighters’ – EU citizens trained in IS terror camps - have returned to Europe and pose a “completely new challenge.”

An April report in the Swedish media said Islamists have successfully infiltrated Sweden’s Green Party. According Lars Nicander from the Swedish National Defence University, there is “a very similar effect today in which people close to the Muslim Brotherhood, an Islamist party, are apparently gaining large footholds in the Green Party.”