OSLO, Norway — Five teams have withdrawn from one of the world’s biggest youth soccer tournaments being held in Norway today after police said the country may be the target of an imminent terrorist attack.

The organizers of the event, which will host children from more than 100 nations, said the teams left out of concern for their safety, according to broadcaster NRK. Owners of the Norway Cup have coordinated security with police after the nation’s intelligence services said July 24 they had received information that Islamist extremists returning from Syria may be plotting an attack.

By Sunday evening in Norway, police reduced the official threat level.

“The threat of a terror attack against Norway is somewhat reduced,” Benedicte Bjoernland, head of security service PST, said today in an Oslo press briefing. “The situation is still serious, and it´s not clarified. We´re now working with great intensity to get a clarification.”

Police in Scandinavia’s richest nation per capita are investigating whether 15 residents who fought in Syria’s three-year-old civil war have been in contact with people expected to return to Norway to commit acts of terror, Jon Fitje Hoffmann, a director at Norway’s security services, PST, told radio broadcaster NRK Saturday.

The terror alert follows a May attack at the Jewish Museum of Belgium in Brussels by a man authorities said may be the first European jihadist returning from Syria to carry out a deadly mission at home. The alleged perpetrator, Mehdi Nemmouche, is awaiting trial for the shooting deaths of two Israeli tourists, a French woman and a Belgian museum employee.

Norwegian police estimate that about 50 people seen as posing a threat have left Norway to fight in Syria, and about half of those have since returned, spokesman Trond Hugubakken said July 24. 

Police officers in Oslo are stationed at focal points in the city including parliament and the royal palace as well as at shopping centers, spokesman Kaare Hansen said by phone. Authorities have followed up on a number of tips, police said, without providing more details.

In Bergen on the country’s west coast, all airspace was closed over the city center Saturday, Hordaland police security chief Roald Eliassen said in an interview with TV2. Norway’s Jewish museums in Oslo and Trondheim closed July 25 on concern they may be targeted in any attack, while the U.S. embassy in the capital warned citizens to remain cautious.

More than 170,000 people have died and over 10 million have fled their homes since civil war broke out in Syria in March 2011. The United Nations and aid agencies say the conflict is the worst humanitarian disaster since the 1994 Rwandan genocide, with 6.5 million people displaced inside Syria and 3 million more seeking refuge outside the country.

News of the latest threat to Norway came just two days after the country marked the third anniversary of the massacre of 77 people, most of them linked to the Labor Party that was in government at the time, by Anders Behring Breivik. The 35-year- old, who is serving a 21-year prison sentence, has said his acts were meant to prevent the spread of what he called “cultural Marxism” and the “Islamization” of Europe.