The Norwegian government is to meet to today to discuss security measures after the twin attacks which killed over 90 people in the country on Friday, Prime Minister Jens Stoltenberg told a news conference on Saturday.

"So far there has been no basis for raising the threat level," Stoltenberg said.

At least 80 people were killed in a shooting at a youth summer camp on Utoya Island near Oslo, police said.

A further seven people were killed in a bomb attack on a government building in Oslo. The mayor's office told CNN about 90 people were also injured in that attack.

A 32-year-old Norwegian who opened fire at the youth camp has been arrested. He is also believed to have been behind the Oslo bombing.

The man, an Oslo resident, is believed to belong to right-extremist groups in eastern Norway. He is unlikely to be linked to any international terrorist organizations, police said.

"The country has not experienced the crimes of this scale since the Second World War," Stoltenberg said.

Russian President Dmitry Medvedev has expressed his condolences in a message to King Harald V of Norway and to Stoltenberg, the Kremlin press office said.

"I am sure, those who organized and carried out these merciless and senseless crimes will be held liable and will be punished appropriately," Medvedev said in a message.