CIA director John Brennan warned that the attacks in Paris were likely not a "one off event" and that he expects the Islamic State group has more operations in the pipeline.
Blasts shook the Islamic State group's Syrian stronghold Raqa as French air strikes hit weapons depots and a training camp in response to the attacks in Paris.
British security services have foiled around seven terror attacks since June with fighters returning from Syria posing a growing threat.
French warplanes pounded Islamic State's Syrian stronghold in retaliation for a wave of coordinated attacks claimed by the jihadists which left 129 people dead in Paris.
Up to 20,000 people, among them Prime Minister Lars Lokke Rasmussen, gathered for a candlelight vigil at the gates of the French embassy in Copenhagen mourn the victims of the Paris attacks.
The defense ministers of France and the United States agreed on "concrete steps" to intensify cooperation against the Islamic State group, the Pentagon said.
German President Joachim Gauck said the world is confronted with a "new type of war" perpetrated by terrorists bent on killing those who refuse to follow their "barbaric ideology".
Moscow called on Saturday for the international community to unite in its fight against terrorism after a string of attacks in Paris left more than 128 people dead on Friday.
British Prime Minister David Cameron said he was "shocked" after at least 18 people were reported killed in multiple attacks in Paris, including one near the Stade de France sports stadium.
UN Secretary-General Ban Ki-moon called for a "unified" global response to prevent terrorism that puts respect for human rights at the "forefront".
Democratic presidential frontrunner Hillary Clinton faces a congressional grilling over her role during and after the deadly attacks on the US mission in Benghazi.
Anger towards President Recep Tayyip Erdogan over Turkey's worst-ever terrorist attack intensified Sunday as authorities raced to identify the two male suicide bombers it blamed for the bloodshed.
Hundreds of people waving American flags lined the streets of Sacramento to pay tribute to three local heroes who helped thwart a train attack in France.
Turkish Prime Minister Ahmet Davutoglu vowed to "wipe out" Kurdistan Workers' Party (PKK) militants from their mountain strongholds after 16 soldiers were killed in a rebel attack.
National Guardsman Alek Skarlatos, one of three Americans hailed as heroes, has joined the cast of "Dancing with the Stars."
Interior and transport ministers from EU countries are to hold talks in Paris to discuss security measures following last week's attack on a high-speed train that was foiled by passengers.
President Francois Hollande warned that France must brace for further attacks, just days after a potential bloodbath on a train was foiled by passengers.
The US serviceman who was wounded while foiling a gun attack on a train in Europe will be nominated for the Air Force's highest medal for non-combat bravery.
An unknown man assaulted a currency exchange office in Astana on August 17. Police made an arrest the same day.
Three American friends who overpowered a heavily-armed gunman in a train poured cold water on his claim that he had no intention of waging a bloody attack.