Western leaders stepped up the rhetoric against the Islamic State group on Sunday as residents of the Belgian capital awoke to largely empty streets and the city entered its second day under the highest threat level.

With a menace of Paris-style attacks against Brussels and a missing suspect in the deadly Nov. 13 attacks in France last spotted crossing into Belgium, the city kept subways and underground trams closed for a second day. Officials also recommended that sports competitions and all activities in public buildings be cancelled and malls and commercial centers closed.

Western leaders, meanwhile, vowed to stand up to IS, which has claimed responsibility for the attacks in Paris that killed 130 people and wounded hundreds more, the suicide bombings in Beirut that killed 43 people and injured more than 200, and the downing of the Russian airline carrying 224 people in Sinai. All happened within the past month.

Speaking from Kuala Lumpur, President Barack Obama said the world would not accept the extremists' attacks on civilians as the "new normal," and vowed the United States and its international partners would not relent in the fight against the Islamic militants.

French Defense Minister Jean-Yves Le Drian said IS must be destroyed at all costs.

"We must annihilate Islamic State worldwide ... and we must destroy Islamic State on its own territory," Le Drian said, speaking on Sunday morning news shows. "That's the only possible direction."

France has intensified its aerial bombing in Syria and Le Drian said French aircraft carrier Charles de Gaulle, which has been sent to help operations against IS militants in Syria, will be "operational" from Monday and "ready to act."

French President Francois Hollande is scheduled to meet in Paris with British Prime Minister David Cameron on Monday, and will travel to Washington and Moscow later in the week to push for a stronger international coalition against IS. Cameron is expected to outline his plan for combating the Islamic militants this week as he seeks Parliamentary approval to join France, the United States and Russia in striking the group's strongholds in Syria.

Tensions have been high in Europe since the attacks in Paris. France has extended a state of emergency — which allows police raids, searches and house arrest without permission from a judge — for three months, and on Saturday extended a ban on demonstrations and other gatherings through Nov. 30, when a U.N. climate conference with more than 100 heads of state is scheduled to start.

All the usual markets in Brussels were canceled Sunday and the minister of education, Joelle Milquet, told Le Soir newspaper that there was a 50 percent chance schools would be closed Monday.

Bernard Clerfayt, mayor of the Brussels suburb of Schaerbeek, told Belgian media that the Belgian capital was still under a grave threat. He said as long as this threat was present, it was essential to remain very vigilant.

Several of the Paris attackers had lived in Brussels, including suspected mastermind Abdelhamid Abaaoud, who was killed in a standoff with French police on Wednesday.

Salah Abdeslam, another suspected attacker, is at large and is known to have crossed into Belgium the morning after the attacks. And in the Turkish coastal city of Antalya, authorities detained a 26-year-old Belgian citizen, suspected of being connected to Islamic extremists and possibly to the Paris attacks — raising hopes of a possible new break in the investigation.

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Sanminiatelli reported from Paris, Thomas Adamson and Frank Jordans in Paris, and Josh Lederman in Kuala Lumpur, Malaysia, contributed to this story.