Investigators say clear links have emerged between the jihadists involved in the Paris and Brussels attacks, suggesting a single cell was responsible for both.

WHO IS STILL SOUGHT OVER BRUSSELS ATTACKS?

NATO is ready to step up cooperation with Moscow to combat terrorism, but rejects any "trade-off" to abandon a military buildup on its eastern flank facing Russia, a senior alliance official said Thursday.

NATO Deputy Secretary General Alexander Vershbow said all allies were willing "to cooperate more with Russia against terrorism" but that would not change plans to deter military resurgent Moscow.

The search grew increasingly desperate on Thursday for loved ones still missing after the Brussels attacks as relatives and friends clung to fading hopes of hearing good news.

Tuesday's attacks at Brussels airport and at a metro station in the Belgian capital killed 31 people and injured 300, 61 of whom were in critical condition.

Belgian ministers under fire for intelligence failings over the deadly Islamic State suicide attacks on Brussels admitted "errors" and offered to resign Thursday as police hunted two suspects still at large after the bombings.

With criticism growing that international security authorities failed to follow links between Tuesday's bombings and similar attacks that hit France in November, key Paris suspect Salah Abdeslam insisted he was unaware of plans to strike the Belgian capital.

The Brussels attackers included a pair of brothers, the latest in a long string of sibling duos involved in violent religious extremism -- something experts say can occur when close childhood bonds "spin out of control."

"It's a completely natural phenomenon," said Marc Sageman, a psychiatrist and former CIA agent who was among the first to identify it in his 2003 book "Understanding Terror Networks".

The two brothers who blew themselves up in the deadly train and airport attacks in Brussels were known by police for serious crimes and are linked to November's Paris massacre.

Khalid and Ibrahim El Bakraoui are Belgian nationals with major convictions "not linked to terrorism", federal prosecutor Frederic Van Leeuw said at a dramatic news briefing on Wednesday.

Israel's intelligence minister accused Belgian leaders of laxity Wednesday over the threat posed by homegrown Muslim radicals, the second cabinet member to hit out after the deadly bombings in Brussels.

"If in Belgium they continue to eat chocolate, enjoy life and parade as great liberals and democrats while not taking account of the fact that some of the Muslims who are there are organizing acts of terror, they will not be able to fight against them," Yisrael Katz told public radio.

They came from Peru or Morocco, from North America or Europe, and their lives -- as parent, eurocrat, sportsman or missionary -- were just as diverse.

That is the emerging picture of the hundreds of people fated to be killed or wounded in the triple bomb attack in Brussels on Tuesday.

French Finance Minister Michel Sapin was heavily criticized on Wednesday after he accused Belgian authorities of "naivety" over the spread of Islamist extremism.

"I think there was a will, or a lack of will, on the part of some (Belgian) authorities... perhaps also a kind of naivety," Sapin said Tuesday, suggesting that they "thought that to encourage good integration, communities should be left to develop on their own."

President Barack Obama condemned the "outrageous" attacks Tuesday in Brussels that killed about 35 people, saying the United States would do everything in its power to hunt down those responsible.

"We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism. We can and we will defeat those who threaten the safety and security of people all around the world," he said, speaking in the Cuban capital Havana.