With Norway involved in both the Afghan and Libya campaigns, NATO's chief said the alliance stood united against the "heinous" acts of violence while US President Barack Obama urged countries around the world to step up cooperation in anti-terrorism efforts.
The attacks were "a reminder that the entire international community has a stake in preventing this kind of terror from occurring," Obama said during a meeting with New Zealand's Prime Minister John Key.
"We have to work cooperatively together on intelligence and in terms of prevention of these kinds of horrible attacks," added Obama after receiving a briefing on the attacks from his top anti-terrorism adviser John Brennan.
NATO's Secretary General Anders Fogh Rasmussen said the alliance was united in its repugnance at the "heinous acts of violence in Norway".
"Our solidarity with Norway remains steadfast. NATO countries stand united in the battle against these acts of violence," he added.
Norwegian combat jets have conducted bombings in the air war against Muammar Gaddafi's regime in Libya, although it is withdrawing the warplanes at the end of the month.
The Norwegian military said in May that it had been the victim of a serious cyber attack at the end of March on the day after its jets for the first time carried out bombings in Libya. Norway also has around 500 military personnel in Afghanistan, primarily in Kabul and in the north.
The condemnations from Rasmussen and Obama were echoed by Herman Van Rompuy, the president of the European Union, who spoke of his "deep shock" at the killings.
"I condemn in the strongest terms these acts of cowardice for which there is no justification."
Van Rompuy said he sent a message of "condolences and solidarity" from the 27-nation EU to Prime Minister Jens Stoltenberg.
European Commission president Jose Manuel Barroso voiced shock at the "abhorrent images" of the bomb blast in Oslo's government quarter.
"An attack of this magnitude is not something one would expect in Norway, famously associated with peace at home and peace-making abroad," Barroso said.
Carl Bildt, foreign minister of neighbouring Sweden, expressed solidarity with his fellow Scandinavians.
"Terror has struck. We are all Norwegians," he said in a message on Twitter.
British Foreign Secretary William Hague also offered his condolences to all those bereaved or injured as a result of a "horrific" bomb attack in Oslo, saying he condemned all forms of terrorism.
"The UK stands shoulder to shoulder with Norway and all our international allies in the face of such atrocities. We are committed to work tirelessly with them to combat the threat from terrorism in all its forms," he said.
French President Nicolas Sarkozy condemned what he said was an "odious and unacceptable act" of violence while Dutch Prime Minister Mark Rutte said that the attackers had "shown a total lack of respect for human life".
And in Berlin, German Chancellor Angela Merkel said she was "horrified". Her "government and the German people are standing by" Norway, she added.