22 July 2011 Last updated at 15:48 ET 
Police say they believe the explosion is linked to the later shooting
Prime Minister David Cameron has said he was "outraged" at the attacks in Norway which have reportedly left at least 11 people dead.
He said he has spoken to the country's PM Jens Stoltenberg to express the UK's condolences and offered assistance in tracking down the perpetrators.
A bombing in Oslo severely damaged government offices and several people are reported dead at a youth camp.
Diplomats say they are checking whether any British nationals are affected.
Mr Cameron said: "My thoughts are with the wounded and those who have lost friends and family, and I know everyone in Britain will feel the same.
"These attacks are a stark reminder of the threat we all face from terrorism.
"I have called Prime Minister Stoltenberg this evening to express my sincere condolences and to let him know that our thoughts are with the Norwegian people at this tragic time.
"I have offered Britain's help, including through our close intelligence cooperation."
The UK's ambassador to Norway, Jane Owen, told the BBC: "We are in touch with the Norwegian government at all levels and also with the police who are trying to work very hard to establish what has happened.
"We are also in very close touch with hospitals and other services to try and establish whether there are any British citizens affected we can help."
She added it was believed to have been the first attack of its kind in the country but said: "The Oslo government, however, has been aware for a long time of the potential danger and threat and have been working to combat it. And we indeed have been working very closely with them on the terrorism agenda."
Embassy assistanceAround 250,000 British tourists visit Norway every year, the Foreign Office says.
Advice on the Foreign Office website recommends that British nationals currently in Norway should stay indoors for the time being.
Continue reading the main story"Start Quote
End Quote William Hague Foreign SecretaryOur embassy stands ready to provide assistance to any British nationals who may have been caught up in the attack"
"British nationals are advised to exercise caution, monitor local media reporting and follow local advice given by the emergency services," it says.
Shortly after news of the shooting emerged, Norwegian media said at least four people were killed when a man opened fire indiscriminately at a Labour Party youth camp in Utoeya, an island outside the capital.
No group has said they carried out the two attacks but police say they believe the bomb blast and the youth camp shootings to be linked.
Foreign Secretary William Hague said Britain stood "shoulder-to-shoulder with Norway and all our international allies" after the "horrific" attack.
"Our embassy stands ready to provide assistance to any British nationals who may have been caught up in the attack."
Scotland's finance secretary John Swinney also sent a message of condolence to Norway.
He said: "I have spoken with David Windmill, the honorary consul general, this afternoon and conveyed on behalf of the Scottish government our deepest sympathy and condolences."
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