The desperate manhunt for terrorists involved in the Paris atrocities will see Brussels in lock-down for a third day as the international community prepares to intensify action against Islamic State.
Belgian prime minister Charles Michel said schools, universities and the underground system would have to remain closed tomorrow amid fears of a repeat of the simultaneous gun and bomb attacks in the French capital.
Key suspect Salah Abdeslam is just one of several feared to be at large in the city, where the usually thronged streets have been eerily quiet after the country was moved to its highest level of security alert.
"We fear an attack like in Paris, with several individuals, perhaps in several places," Mr Michel warned after a meeting of the country's national security council.
As the search for the jihadists continued, world leaders were contemplating how to respond to a string of outrages, with the UK moving nearer to joining allied air strikes against IS targets in Syria.
David Cameron will join French president Francois Hollande in Paris tomorrow morning to discuss the crisis and the role British forces can play in the offensive against the extremist strongholds.
The PM is to present his case for escalating British military involvement to Parliament later this week - with the Paris attacks and an unanimous UN Security Council resolution apparently galvanising support among MPs for the move.
Barack Obama pointed to an "increasing awareness" by Russian president Vladimir Putin of the need to eradicate the threat from IS as efforts continued to forge a co-ordinated response.
The US president also urged leaders "to send a signal that the viciousness of a handful of killers does not stop the world from doing vital business".
With Brussels on the highest Level 4 alert, officials recommended the cancellation of sports competitions and the closure of shopping centres and public buildings.
The Great Britain tennis team has delayed its departure for next week's Davis Cup final, due to be played at the 13,000-seat Flanders Expo in Ghent, which is only 35 miles north-east of Brussels.
More than 1,000 British fans are expected to support the team but the Foreign Office is advising visitors to Belgium against places with large numbers of people.
In Paris, French police issued a photo of the as-yet-unidentified third attacker who died outside the Stade de France stadium.
It is believed a Commons vote on air strikes could be held as early as next week and Chancellor George Osborne said the deaths of 130 on the streets of Paris and the UN resolution backing "all necessary measures" were swaying the argument.
Warning that defeat in the vote would be "a publicity coup" for IS, he said it was now clear there was "a price for not getting involved".
"The Prime Minister will seek support across Parliament for strikes against that terrorist organisation in Syria. Frankly, Britain has never been a country that stands on the sidelines and relies on others to defend us," he said.
"We'll make the case as a Government, we will allow MPs to digest that response and then we will see where we stand."
Mr Cameron's case will come in the form of a response to a report by the Commons foreign affairs committee which expressed severe reservations about the coherence of the Government's case.
Its Conservative chair Crispin Blunt has indicated the conditions set in the report could now be met.
Labour leader Jeremy Corbyn has said the UN vote should be a catalyst for renewed efforts to find a political solution in Syria, not "external intervention", but he faces demands from Labour MPs not to enforce a party line against air strikes.
Dozens are reported to be ready to defy him in the Commons.
Democratic Unionist Party deputy leader Nigel Dodds signalled its eight MPs were willing to back air strikes.
Scottish National Party deputy leader Stewart Hosie sounded a more cautious note however - insisting that the Prime Minister would need at least to signal his intention to seek a UN resolution specifically authorising military action before it would consider voting in favour.
Conservative MP Andrew Percy, one of 30 who voted against air strikes in Syria in 2013, suggested he could be won over this time.
Canvassing the opinion of constituents in Brigg and Goole, he wrote on Twitter: "News reporting vote on Syria is coming. I opposed last time and still need convincing but coming round to it."