Denmark will not allow the return of about 30 children whose parents travelled to Syria and Iraq to fight for ISIS, the country’s prime minister said.

Mette Frederiksen, the leader of the left-wing coalition government, said allowing children into Denmark would open the door to their parents.

The UN estimates there are about 30,000 children of foreign fighters in Syria and Iraq. Two thirds of them are Iraqi children in Syria.

“The challenge is that we cannot separate the children from their mothers,” Ms Frederiksen told the Danish news agency Ritzau.

There were no known cases where children of Danish foreign fighters had been allowed to return to the country.

Ms Frederiksen said she had been repeatedly urged to take only the children back.

“No, I can’t, because then their mothers come along and they don’t have a place in our society,” she said.

The Danish government said 36 extremists were known to have travelled from Denmark to combat zones while about 30 of their children were believed to be in the region.

Ms Frederiksen became prime minister of a minority government in June with a promise to impose strict controls on refugees and asylum seekers.

Her promise undercut the vote for the anti-migrant Danish People’s Party, support for which was slashed.

The government has pressed ahead with its own tough policies. Parliament passed a law in October allowing the government to strip the passports from Danes who travelled abroad to join militant groups.

The following month, two men had their passports seized.

The policy follows tough measures introduced by previous governments, which included pulling out of a UN resettlement programme and a 2016 law that allowed authorities to seize valuables from migrants to help finance the costs of their stay.

Denmark was also criticised by rights groups this month after authorities rejected the asylum claims of three Syrian women, saying it was safe for them to return to Damascus.

Denmark among European countries grappling with the issue of returning foreign fighters and the threat of terrorism from supporters of ISIS.

Prosecutors have warned that the difficulty of obtaining evidence in war zones would make it hard to secure terrorism convictions.

They are also concerned about the prospect of hardened fighters returning to sow discord and launch attacks in their own countries.

Twenty-one people were arrested this month in Denmark, including a group who were accused of trying to obtain ingredients to make explosives.

Another group was stopped from buying a cache of guns and ammunition to launch a terrorist attack. Authorities said the suspects had a “militant, Islamist motive”.

In 2015, two people were killed and six police officers wounded in a terror attack that began outside a culture centre hosting a freedom of speech event in Copenhagen.

The gunman was killed by police.