The international fight against Islamic State moved up a gear on Saturday as Turkish police arrested a Belgian suspect heading for Syria in connection with last week’s Paris attacks, hours after the UN security council unanimously urged all its members to combat the extremist group.

A day after the EU imposed tough new external border controls, authorities in Belgium raised the alert level in the capital to level four – signifying a “serious and imminent threat” – as the hunt for the remaining jihadi who killed 130 people in Paris entered its second week.

The Belgian prime minister, Charles Michel, said the decision to raise his country’s terror threat was taken “based on quite precise information about the risk of an attack like the one that happened in Paris” and a fear that “several individuals with arms and explosives could launch an attack … perhaps even in several places”. Belgian officials said police had discovered a cache of weapons and ammunition in a building in the small Brussels borough of Molenbeek during searches on Friday night.

Several members of the Isis cell of gunmen and suicide bombers who attacked the Stade de France, the Bataclan and a string of cafes and restaurants on 13 November – at least one of whom is still on the run – lived in Molenbeek, an area with a longstanding reputation as a hotbed of extremism.

The highest number of foreign fighters from an EU country – in per capita terms – comes from Belgium and Molenbeek is where they are concentrated.

The interior minister, Jan Jambon, a Flemish nationalist, said that of 130 foreign fighters who were known to have returned home from Syria, 85 were living in Molenbeek just west of the city centre. He demanded that the local authorities conducted a “door-by-door” vetting of who was living in every house and flat in the borough.

The presumed ringleader of the Paris attacks, Abdelhamid Abaaoud, a 28-year-old Belgian national of Moroccan origin, was thought to have been in Syria but died in a ferocious seven-hour gunfight with police at a rundown apartment in St-Denis, north of Paris, on Wednesday.

Another suspected member of the unit, Salah Abdelslam, a 26-year-old Frenchman living in Brussels, is the subject of an international manhunt. His brother, Brahim, who blew himself up in the attacks, spent time in a Belgian prison with Abaaoud, while a third Molenbeek resident, Bilal Hadfi, was also among the Paris suicide bombers.

Belgian special forces arrested four people in the capital’s Place du Grand Sablonon Saturday afternoon, while the Brussels Metro system is expected to be shut down until Sunday afternoon at the earliest. Shops were closed, some shopping malls were shuttered, professional football was cancelled, concerts were called off and music venues, museums, and galleries shut their doors for the weekend.

While local supermarkets were as busy as ever and open-air markets operated normally, there was an edginess in the city centre shopping area, with security personnel posted at the entrance to shops, checking bags and rucksacks on entry. By the afternoon, around half of city centre shops had closed. Armed personnel carriers and armoured vehicles restricted movement in the old city centre.

Municipal facilities across the city, such as sports and arts centres, libraries, and swimming pools, were all ordered to close. The Sunday morning market at the Gare du Midi, the Eurostar terminal – one of the biggest outdoor markets in Europe – was called off.

The US embassy advised Americans in Brussels – home to Nato headquarters – to cancel any plans they had for the weekend and to stay at home. “If you must go out, avoid large crowds. US citizens are urged to avoid public places,” the embassy said.

“If you were planning to attend an event, we strongly urge you to reconsider. Exercise caution in public transportation systems, sporting events, residential areas, business offices, hotels, clubs, restaurants, places of worship, schools, public areas, shopping malls and other tourist destinations.”

The Foreign Office delivered similar advice for the high numbers of Britons in Brussels.

The security alert came after Belgian authorities charged a third man with terrorist offences in connection with the Paris attacks. Earlier this week, two Belgian nationals of Moroccan origin, Hamza Attou and Mohammed Amri, were charged after admitting to driving Abdeslam back to Belgium on the night of the attacks.

Confirmation that Abaaoud – who openly boasted in Isis propaganda of his ability to move at will between Syria and Europe – had managed to make his way to the French capital unnoticed, despite being the subject of an international arrest warrant, forced EU ministers to rush through tougher controls at the borders of the passport-free Schengen zone.

The French interior minister, Bernard Cazeneuve, said the new measures, which require obligatory, systematic and coordinated checks on all travellers entering the 26-nation zone, would be introduced immediately, with the European commission expected to formalise the new rules by the end of the year.

The change means EU citizens, who were previously subjected only to minimum controls when entering the Schengen area, will have their details thoroughly checked against national and international crime and terrorism databases.

Malian policemen and security officials take up positions in a side street to the Radisson hotel where extremists took hostages. Photograph: UPI /Landov/Barcroft Media

As police forces across Europe widened their search for the extremists’ accomplices, Ahmet Dahmani, a 26-year-old Belgian national of Moroccan origin was detained at a luxury hotel in Antalya, Turkey, on suspicion of having scouted out the target sites for the Paris attacks.

Two other men, both Syrian nationals, were also arrested, a government official said. They are suspected of being Isis recruits sent to accompany the alleged scout over the border to Syria. The official said that Dahmani had arrived from Amsterdam on 14 November, the day after the Paris attacks, but had not been intercepted earlier because Turkey had received no warning about him.

At the UN, the security council unanimously called on all able states to join the fight against Isis in Syria and Iraq, and redouble their efforts to prevent any further attacks by the militant group.

The 15 members of the council adopted a French-drafted resolution on Friday calling on UN members to take “all necessary measures” against a group it described as “a global and unprecedented threat to international peace and security”. Countries were urged to step up sanctions and improve efforts to cut off the flow of foreign fighters to Iraq and Syria.

Russian warplanes again struck jihadi targets in Syria overnight, firing cruise missiles from warships in the Caspian Sea into Isis-controlled territory in what Moscow called an “aerial campaign of retribution” for the deaths of all 224 passengers in a Russian plane over Sinai this month.

Rami Abdel Rahmandurrahman, of the Syrian Observatory for Human Rights monitoring group, told AFP that the 70-plus overnight raids were “the worst bombardment of the region since the start of the uprising in 2011”.

In Paris, it emerged on Friday that Abaaoud’s cousin, Hasna Aitboulahcen, who also died in the St-Denis police siege, had not blown herself up as originally thought but had died when the suicide vest of a third, as yet unidentified extremist present in the apartment, was detonated.

Seven of the eight people who were arrested during the St-Denis shootout were released on Saturday, the Paris public prosecutor’s office said. The eighth, Jawad Bendaoud, remains in custody.