Blunt assessments by Barack Obama of longtime U.S. ally Saudi Arabia have triggered unprecedented Saudi criticism of the president as he prepares to visit for a key summit with Gulf allies next month.

Obama's comments, published in the April edition of U.S. magazine The Atlantic, have met with a chorus of outrage across the kingdom's tightly controlled media and the pan-Arab newspapers it owns.

As Brussels reels from an attack by Islamic State jihadists, analysts warn of a ripple effect that could further whip up populist sentiment on the continent and in the United States.

Across the Atlantic Ocean, Tuesday's bombings in the Belgian capital have provided fodder for Donald Trump's divisive electoral campaign while in Europe they risk hardening responses to the refugee crisis.

A year after it launched air strikes in Yemen, a Saudi-led military coalition has failed to deal a decisive blow to Iran-backed rebels and is facing mounting criticism over civilian casualties.

With warring parties drained by the fighting, the United Nations said Wednesday that a ceasefire had been agreed from April 10, to be followed by peace talks.

Jihadist cells like the one that carried out the Brussels attacks are supported by the Islamic State group's leadership in the Middle East, but are choosing themselves where and when to strike, experts say.

And that degree of autonomy is making them all the more difficult to track, and doubly dangerous.

More than 30 people have been identified as being involved in a network behind the Paris attacks on November 13, with links now established to this week's bombings in Brussels.

This is what we know so far about the attackers and their support network.

Terrorism will cast a continuing shadow over future generations and government electronic surveillance is a small price to pay to combat it, a leading historian said Wednesday, a day after the carnage in Brussels.

British author and journalist Sir Max Hastings gave a robust defense of electronic intelligence-gathering in what he called a new world that would never know absolute security.

Brussels has become infamous as a hotbed of Islamic extremism because of links to a series of recent attacks in Europe, and now the Belgian capital itself has suffered the worst ever terror attack in its history.

The attacks on the Brussels airport and metro system which killed around 35 people on Tuesday came just days after Salah Abdeslam, a key suspect in November's Paris attacks, was captured in the city after four months on the run.

The carnage unleashed in Brussels on Tuesday shows that jihadist networks in Belgium and across Europe are still capable of staging mass-casualty attacks despite an intensifying security crackdown, experts say.

A senior French counter-terrorism official said the attacks were unlikely to be a direct response to the arrest in Brussels just four days ago of Saleh Abdeslam, suspected of being the last surviving member of the jihadist team that struck Paris in November.

Tintin wept.

Deadly attacks Tuesday at the Brussels airport and a metro station in the city are the latest in a string of attacks in Europe in recent years. Here are some of the most recent major ones:

— Nov. 13, 2015: Islamic State-linked extremists attack the Bataclan concert hall and other sites across Paris, killing 130 people. A key suspect in the attack, 26-year-old Salah Abdeslam, is arrested in Brussels on March 18, 2016.