It was unclear when, where, or how they were killed, but their heads were bloodied by apparent gunshot wounds.
Their deaths come days after 129 people were killed in Paris in the worst ever militant attacks on French soil, and as Moscow launched fresh air raids against IS in Syria after confirming a "terrorist attack" brought down a Russian passenger jet in Egypt last month.
The Norwegian prime minister's office said the IS photos "seem to show that the hostage Ole-Johan Grimsgaard-Ofstad was executed. We are still verifying it."
China confirmed Fan’s "inhuman" death in a statement on its website Thursday, vowing to bring his killers to justice.
"The terrorist organisation has no regard for human decency and the bottom line of morality," spokesman Hong Lei said.
"They still carried out their barbaric acts of violence. The Chinese government strongly condemns this inhuman atrocity; we must bring these criminals to justice."
The two men were last featured in Dabiq's September edition, in which IS published an "advertisement" that they were "for sale".
The magazine also featured an article entitled "Paradigm Shift II" allegedly penned by British hostage John Cantlie as a sequel to the last piece published under his name in Dabiq's March issue.
The essay claims media outlets, security services and experts are acknowledging IS "is a genuine state", and comes after a long absence of Cantlie's "work", which is regularly featured in the magazine.
- IS in 'civilian homes' -
Since Sunday, Russian and French raids have struck arms depots, barracks and other areas in Raqa city, the jihadists' stronghold in northern Syria.
"This is where we must hit Daesh, in its lifeblood," said French Defence Minister Jean-Yves Le Drian, using the Arabic acronym for the group.
A preliminary death toll from the Syrian Observatory for Human Rights monitor said 72 hours of strikes "left 33 dead and dozens wounded in IS ranks".
Russia said its air force had destroyed some 500 fuel trucks in the past few days as they transported oil from Syria to refineries in Iraq, a key part of IS financing.
Russia on Wednesday also submitted a revised draft UN resolution calling for closer international cooperation against IS in Syria, parts of which the jihadist group rules under its self-proclaimed "caliphate" that also straddles Iraq.
Aktham Alwany, a journalist and activist from Raqa, said civilians in the city were "only moving around when necessary" out of fear of strikes by "whichever nationality -- Russian, regime, coalition".
"Unfortunately, it's no secret that IS's bases are inside civilian homes. There are some bases that look like they're for IS, but in reality they're empty fakes, while civilian homes are teeming with them," Alwany told AFP.
Raqa was Syria's first provincial capital lost by the government, seized by rebels in 2013 then overrun by IS in January last year. At least 300,000 people live there now, according to analyst Fabrice Balanche.
- Syria not 'natural incubator' -
France began air strikes targeting IS in Syria in September as part of a US-led coalition, while Moscow launched its own air war in Syria, in coordination with President Bashar al-Assad, on September 30.
The US and France have been firm backers of Syria's uprising, while Russia and Iran remain staunch allies of the president.
On Wednesday, Assad said his country was not a natural breeding ground for IS, blaming the West and other Middle Eastern countries for creating the jihadist organisation.
"I can tell you Daesh doesn't have the natural incubator, social incubator, within Syria," he said in a television interview with Italian national broadcaster Rai.
Jihadists who trained in Syria did so with "the support of the Turks and the Saudis and Qatari, and of course the Western policy that supported the terrorists in different ways", he insisted.
Despite their diametrically opposed stances on Assad, France and Russia agreed to coordinate their military and security services to fight IS after the attacks in Paris and the downing of the Russian airliner.
French Foreign Minister Laurent Fabius said Wednesday the aircraft carrier Charles de Gaulle would be operational in the eastern Mediterranean "by the end of the week".
And US President Barack Obama praised Russia as a "constructive partner" in international talks in Vienna aimed at reaching a solution to Syria's bloody conflict, which has cost 250,000 lives over more than four years.