Norway is to enforce a strict minimum age limit on social media of 15 as the government ramped up its campaign against tech companies it says are “pitted against small children’s brains”.

The Norwegian prime minister, Jonas Gahr Støre, conceded it would be “an uphill battle” but said politicians must intervene to protect children from the “power of the algorithms”.

Social media platforms, the Labour leader said, were being misused by the industry and could make users “single-minded and pacified”.

The Scandinavian country already has a minimum age limit of 13 in place. Despite this, more than half of nine-year-olds, 58% of 10-year-olds and 72% of 11-year-olds are on social media, according to research by the Norwegian media authority.

The government has pledged to introduce more safeguards to prevent children from getting around the age restrictions – including amending the Personal Data Act so that social media users must be 15 years old to agree that the platform can handle their personal data, and developing an age verification barrier for social media.

“It sends quite a strong signal,” the prime minister told the newspaper VG on Wednesday. “Children must be protected from harmful content on social media. These are big tech giants pitted against small children’s brains. We know that this is an uphill battle, because there are strong forces here, but it is also where politics is needed.”

While he said he understood that social media could offer lonely children a community, self-expression must not be in the power of algorithms. “On the contrary, it can cause you to become single-minded and pacified, because everything happens so fast on this screen,” he added.

The minister for children and families, Kjersti Toppe, in Stavanger meeting parents campaigning for stricter online regulation for children, said the measure was also intended to help parents. “It is also about giving parents the security to say no. We know that many people really want to say no, but don’t feel they can.”

She said the government was investigating methods of enforcing such restrictions that did not intervene with human rights, such as the requirement for a bank account.

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Australia has also announced a social media ban for younger teenagers and children, saying it would block children from social media and other digital platforms. The age limit has not yet been decided but it is likely to be between 14 and 16.

France is trialling a ban on mobile phones at school for pupils up to the age of 15. If successful, it could be introduced nationwide from January.