France, Britain and other EU nations have initiated steps to thwart potential jihadist reprisals as US-led airstrikes continue on Islamic State fighters in parts of Syria and Iraq.
France's cabinet launched "preventive" measures at public sites and on transport services on Thursday after the beheading of French hostage Herve Gourdel by IS-linked extremists in Algeria.
British police said counter-terrorism units had arrested nine suspected members of a banned organization, mostly in London, including one suspect identified by the British Press Association as the radical preacher Anjem Choudary (pictured in 2012).
In the past, Choudary has denied inciting terrorism.
British Prime Minister David Cameron consulted, meanwhile, with his cabinet ahead of a parliamentary vote on Friday on whether British planes should join the US-led airstrikes against IS.
France carried out another round of airstrikes on presumed IS targets in Iraq on Thursday - its second set of raids using Rafale jets since last Friday.
Almost simultaneously, US, Saudi and Emirati warplanes bombed oil installations in eastern Syria in a bid to cut off a significant source of funding for the IS group.
The Britain-based Syrian Observatory for Human Rights said the US-led strikes killed at least 14 IS fighters and at least 5 civilians.
French flags at half-mast
French President Francois Hollande called for "national unity" as he met with his cabinet and decided to have flags flown at half-mast nationwide, including in Nice, Gourdel's home city.
The mountaineer was abducted on Sunday while hiking in a national park in eastern Algeria. Nice's mayor, Christian Estrosi, said Wednesday's murder of Gourdel by IS-linked Jund al-Khilifa abducters in Algeria was a "terrible shock."
Hollande's office in a statement said "preventive measures against the risk of terrorism will be strengthened at public sites and on transport."
Britain: nine suspects arrested
Britain's Metropolitan Police said the nine suspects were arrested in London on suspicion of being members of a banned organization and "encouraging terrorism."
Police declined to give personal details, but Britain's Press Association and major British news outlets identified the group as the Islamist al-Muhajiroun organization and said one of those arrested was Anjem Choudary. He led the group until it was banned four years ago.
A police statement said 18 premises had been searched in London and one in the central English city of Stoke-on-Trent.
On Wednesday, the UN Security Council passed a resolution demanding that countries take action to stem the flow of foreign jihadists to Iraq and Syria.
On Sunday, IS spokesman Abu Mohammed al-Adnani had issued a recorded 42-minute audio threat, urging "believers" to kill Americans, French, Australians, Canadians "or any other disbelievers" of countries that join the US-led military coalition.
Netherlands: 'higher' threat
The Netherlands government announced on Wednesday that it would deploy six F-16 warplanes for the campaign against IS inside Iraq. Belgium is also set to offer six F-16s, dependent on parliamentary approval. Australia will also send eight fighter jets.
Dutch Deputy Prime Minister Lodewijk Asscher acknowledged that the Netherlands would "gain a higher profile among jihadis."
"We are ready," Asscher said. "The threat profile is monitored permanently and our security services are prepared."
Official terror threat assessments are at the second-highest levels in both the Netherlands and Britain.
Paris has warned 30 of its embassies across the Middle East and Africa of an increased threat.
No uniforms in public
On Thursday, Dutch soldiers were told not to wear uniforms on public transport.
Dutch defense ministry spokeswoman Marloes Visser said "several statements" made earlier this week by suspected radicals were being "monitored."
She denied that the clothing advisory was prompted by the Dutch F-16 announcement.
On Tuesday, a Syria-based Dutch jihadist fighter calling himself Muhajiri called for action against the Netherlands
Dutch state broadcaster NOS said the video voiced in Dutch had apparently been made in the Syrian city of Aleppo.
Germany: Muslims distance themselves
Germany, which is also alarmed by the presence of radicalized nationals in Syria, last Friday promoted a "day of action" during which Muslims distanced themselves from terrorism.
Berlin has said Germany will largely focus on arming and training Kurdish Peshmerga fighters battling IS militants across northern Iraq.
The Muslim Council in France on Thursday called for a rally of French Muslims to take place on Friday against the "barbaric" beheading by IS-linked Algerian jihadists.
ipj/lw (kna, AfP, Reuters, AP, dpa)