- CNN terrorism analyst: "There's a lot of concern" about possible copycat attacks
- Prime minister says France is facing the greatest terrorism threat it ever has
- Australian Prime Minister: "We don't know when and how an attack may come"
An extremist gunman holds hostages for hours in a cafe. A man yells, "God is great," before ramming a car into pedestrians. Attackers with Islamist sympathies ambush soldiers.
Recent "lone wolf" terror attacks in Western nations have some world leaders worrying the threat of another strike could be greater than ever.
"The terror threat remains high ... and at this level an attack is likely. We don't know when and how an attack may come, but we do know that there are people with the intent and the capability to carry out further attacks," Australian Prime Minister Tony Abbott warned on Tuesday, speaking a week after the end of a hostage siege in a Sydney cafe that left two people dead.
"There's a lot of concern now, because we've seen attacks over the past few weeks in the West, that there could be some sort of copycat attack," CNN Terrorism Analyst Paul Cruickshank said.
The threat of terrorist attack in the European Union is greater than at any time since 9/11 because of the emergence of jihadist safe havens in Syria and Iraq, European officials tell CNN.
Could the threat of an attack be greater around the Christmas holiday? Maybe, Cruickshank said.
Documents seized in Osama bin Laden's compound in Abbottabad, Pakistan, suggested al Qaeda might want to strike around Christmas for psychological reasons -- and to affect large numbers of travelers, Cruickshank said.
"But generally speaking," Cruickshank said, "terrorists launch plots when they're ready to go, rather than on a particular date."
Here's a look at some recent terrorist threats, attacks and responses in several Western nations:
It has been more than a week since Man Haron Monis, a self-styled Muslim cleric who had expressed support for ISIS, seized control of the Lindt Chocolate Cafe in Sydney in a high-profile hostage siege that drew global attention.
Since then, Abbott said Tuesday, security agencies have detected a "heightened level of terrorist chatter."
He urged people to be more vigilant, but didn't identify any particular target that could be at risk.
"There has been a heightened level of chatter amongst people who we would normally think of as terrorist sympathizers," he said.
In October, a soldier was shot and killed as he stood guard at Canada's National War Memorial in Ottawa. Officials said the gunman had connections to jihadists who shared a radical Islamist ideology.
Two days before, another Canadian soldier -- this one in Quebec -- was run over and killed by another man whom the Royal Canadian Mounted Police called "radicalized."
"This week's events are a grim reminder that Canada is not immune to the types of terrorist attacks we have seen elsewhere in the world," Prime Minister Stephen Harper said at the time. "Let there be no misunderstanding: We will not be intimidated. Canada will never be intimidated."
Earlier this month, ISIS released a video featuring a Canadian man calling for more "lone wolf" attacks on Canadian targets, CNN partner CBC News reported.
Fears are running high in France after a string of recent incidents.
A man stabbed three police officers in central France on Saturday while allegedly calling out, "Allahu Akbar" -- Arabic for "God is great." French counterterrorism authorities are investigating that attack.
The next day, a man in Dijon called out the same phrase as he rammed a vehicle into pedestrians, injuring at least 12, police said.
And on Monday, another man plowed a van into pedestrians at a Christmas market in Nantes. Officials have said that incident appears to be an isolated case and not an act of terrorism, but Prime Minister Manuel Valls warned Tuesday that there were concerns of possible copycat attacks.
Valls said this week that his country has never faced as serious a terrorism threat as it does now.
"When 1,200 French individuals or residents have links to jihad, when nearly 380 are present in Syria and are active in terrorist groups and learning terrorism and horror ... then of course, let me emphasize, there is a threat of proportions never seen before," he said.
On Tuesday, he announced that France was stepping up security in public places, sending up to 300 additional troops to patrol.
Of all the Western nations, France likely faces the highest threat, Cruickshank said, "with support for ISIS running deep among disenfranchised immigrant communities in the rundown, crime-ridden banlieues that surround many French towns."
Chancellor Angela Merkel has condemned the growing anti-Muslim demonstrations in the German city of Dresden, but some worry that events like the recent attacks in France are likely to further boost their cause.
Thousands of protesters have been meeting on Mondays for the increasingly popular marches.
The latest drew 15,000 people, a loose coalition of anti-immigrant and right-wing groups who say they fear what they call "Islamization."
New York Police Commissioner Bill Bratton said a broad-daylight hatchet assault on a group of New York police officers in October was a "terrorist act" carried out by a self-radicalized convert to Islam.
In the aftermath, some observers questioned if the attack was tied to calls by radicals to strike the military and police, especially on the heels of the attacks on the officers in Canada.
After last week's hostage siege in Australia, the State Department issued a warning saying the attack "is a reminder that U.S. citizens should be extra cautious, maintain a very high level of vigilance, and take appropriate steps to enhance their personal security."
Risks could intensify during the holiday season, the latest worldwide travel alert says.
"An analysis of past attacks and threat reporting strongly suggests a focus by terrorists not only on the targeting of U.S. government facilities," it says, "but also on hotels, shopping areas, places of worship and schools, among other targets during or coinciding with this holiday period."