OSLO, Norway — Police in Norway are on high alert after receiving intelligence that nationals returning from Syria may be plotting a terrorist attack within days against the Scandinavian country.

Information obtained by Norway’s security service, PST, suggests an attack could be imminent, the unit’s chief, Benedicte Bjoernland, said July 24. Authorities have strengthened their presence at Norway’s borders, airports and train stations, and police in all districts are at a heightened state of preparedness.

Police officers in the capital, Oslo, have been 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 received since yesterday, police said, without providing more details.

Norway’s Jewish museums in Oslo and Trondheim closed Friday on concern they may be targeted in any attack, while the U.S. embassy in the capital warned citizens to remain cautious.

The terror threat comes as Norway prepares for a children’s soccer tournament, involving teams from more than 100 nations. The event, which starts tomorrow, has an “extensive” security plan and the organizers are working closely with authorities to ensure safety, according to a statement on the Norway Cup website.

Norway police are investigating whether 15 residents who fought in Syria have been in contact with people expected to return to the Scandinavian nation to commit acts of terror, broadcaster NRK reported today, citing Jon Fitje Hoffmann, a director at PST.

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.

Foreign fighters in Syria who hold U.S. and European Union passports number “in the four digits, so it’s a serious problem,” Michael Vickers, the Pentagon’s top intelligence official, said this week at the annual Security Forum in Aspen, Colorado.

Norwegian police are monitoring individuals who have fought in Syria and are now returning to the Nordic country, Bjoernland said. These combatants are likely to have gained experience using weapons and to have a lower threshold for violence, she said.

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.

The greatest threat to Norway is now posed by extreme Islamists, Bjoernland said.

In May, police arrested three Norwegian citizens on terror charges. The men, one of whom was born in Somalia and the other two in the former Yugoslavia, represented a threat to the country and its allies, police said at the time.

Authorities suspect an attack could occur on July 28 to coincide with the Eid celebration that marks the end of the holy month of Ramadan, TV2 reported, citing unidentified people. Odd Reidar Humlegaard, director of Norway’s police force, declined to comment.

Norway may also be a target after backing the U.S. in Libya and Afghanistan, Elaine Valther Hansen, a senior advisor at the Norwegian Institute of International Affairs, said in an interview with Aftenposten.

The threat is a concern for Norway’s Muslim population, NTB reported, citing Ghulam Sarwar, head of Oslo’s largest mosque. “This is a burden for ordinary Muslims, Muslims are going to be hated by neighbors because of this case,” he said.

There were about 121,000 registered Muslims in Norway at the end of 2013, equivalent to 2.4 percent of the population, according to Statistics Norway.

Norway’s strategy of going public with the terror threat echoes its approach in 1973 when it thwarted a planned attack on an oil installation about 62 miles south of Oslo, Tore Bjoergo, a professor at the Norwegian police academy, said in an interview with Aftenposten.

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.