Europe remained on high alert as soldiers were deployed alongside police in Belgium to protect Jewish buildings and the head of Europol warned that further terrorist attacks could not be ruled out in the wake of the murder of 17 people in Paris by Islamist gunmen last week.

The move to guard potential targets in Antwerp’s Jewish quarter on Saturday was the first time in three decades that Belgian troops have been used to bolster police ranks. It follows the deployment of French soldiers following the attacks in Paris.

Thirteen jihadi suspects were arrested in Belgium on Thursday night in an operation that foiled a sophisticated plot to murder police, authorities said.

Belgium has increased its terror warning to three, the second highest, following the raids in which two suspects were killed.

Rob Wainwright, director of Europol, said that fully containing the threat posed by terrorism in Europe was challenging, adding there was no guarantee further attacks could be prevented.

He told Sky News: “No, there can’t be, otherwise what happened in Paris wouldn’t have happened. I think there is a realisation across the police and security community in Europe.

“But at the same time we have a very strong determination to maximise our capability to keep our citizens safe.”

Authorities in Yemen said on Saturday that two Frenchmen suspected of links to al-Qaida have been detained.

“During the past two days, two French nationals accused of belonging to al-Qaida have been arrested,” the country’s national security service chief, Gen Mohammed al-Ahmadi, told the AFP news agency.

Al-Qaida in the Arabian Peninsula (Aqap) claimed responsibility for the 7 January attacks at the offices of satirical magazine Charlie Hebdo in which 12 people were murdered by French brothers Saïd and Chérif Kouachi.

The pair are known to have trained with al-Qaida in Yemen, which was formed in 2009 after a merger between militants there and Saudi Arabia.

“There are around 1,000 Al-Qaida militants in Yemen from 11 Arab and non-Arab countries,” Ahmadi said.

The terror threat against British police has been raised to severe, the highest level yet, with police chiefs considering the more widespread deployment of Taser stun guns as an additional security measure.

Police patrols of Jewish communities in Britain are also to be stepped up amid heightened concern about the risk of an attack following the Paris atrocity, Britain’s top counter-terror officer said. Extra security measures will be taken at Jewish schools.

The scale of the jihadi threat has increased over the past decade, Wainwright said, because terrorists no longer had a coherent, identifiable command and control structure. “But over those 10 years, the sophistication of the police response has also increased.”

It emerged on Saturday that Saïd Kouachi, who was killed by security forces in a siege, has been buried in secret.

His funeral happened on Friday in Reims, eastern France, where he lived for about two years, under heavy police protection, according to a source.

His grave was unmarked and the name of the cemetery was not released. His wife did not to attend the burial to keep it secret, said her lawyer Antoine Flasaquier. His brother Chérif was expected to be buried soon in the Paris suburb of Gennevilliers.

Plans to bury Amédy Coulibaly, who murdered five people at a kosher supermarket in Paris before being killed by police, remained unclear.

Meanwhile, the French president, François Hollande, said on Saturday that France was committed to freedom of expression after violence erupted for the second day in Niger at a rally protesting against Charlie Hebdo. The magazine’s first issue since last week’s attacks features a drawing of the prophet Muhammad on the cover, in a decision that has angered Muslims around the world.

“I’m thinking of countries where sometimes they don’t understand what freedom of expression is because they have been deprived of it. But also, we have supported these countries in their fight against terrorism,” Hollande said during a visit to a market in Tulle, central France.

Asked about protesters who burned the French flag, the president added: “They have to be punished because when it happens in France, it’s intolerable but also abroad.”

Around 15,000 people also gathered in the town of Magas in Russia’s Muslim north Caucasus region of Ingushetia on Saturday, protesting against western publications that insult the prophet, authorities said.