Japan must ban sexually abusive images of children in manga comics, despite claims that such a move would threaten freedom of expression, the UN’s special envoy on child protection has said.

Maud de Boer-Buquicchio praised Japan for passing a law last year that banned the possession of abusive images of children, but said it contained loopholes that allowed exploitation to continue.

“When it comes to particular, extreme child pornographic content, manga should be banned,” De Boer-Buquicchio said at the end of a week-long visit to Japan.

The UN’s special rapporteur on the sale of children, child prostitution and child pornography, acknowledged that artists and publishers faced difficulty in “finding the right balance” between artistic freedoms and the need to protect children. “I accept that the freedom of expression argument should prevail when it comes to adult pornography.”

In June 2014, Japan banned the possession of child sexual abuse imagery after years of delay, but disappointed campaigners by not including the multibillion-yen market in manga comics, animated films and video games.

There was a grace period of a year to comply, but lawbreakers now face prison terms of up to a year and fines of up to 1m yen (£5,500).

Previously, Japan had been the only G7 country where it was legal to own videos, photographs and other imagery depicting sexual crimes against children, provided there was no intention to sell them or post them on the internet.

Japan’s inaction prompted criticism from overseas, with humantrafficking.org describing the country as “an international hub for the production and trafficking of child pornography”.

Among the actual images of children not covered by the new law are photographs of children in skimpy outfits but with their genitalia concealed. The material is easily available in DVD and photo book form in Tokyo.

“All these are obviously lucrative businesses. What is worrying is that there is a trend which seems to be socially accepted and tolerated,” De Boer-Buquicchio said.

According to Japanese police, the number of under-18s who featured in sexually abusive images rose to a record high of 383 in the first half of this year, up 58 from the year before. Action was taken against 659 people in 831 cases, the highest numbers since twice-yearly records began in 2000, according to a preliminary report issued by the national police agency last month. About 90% of the victims were female, including 60 who were of elementary school age or younger.

Manga artists and publishers have long resisted government attempts to ban certain images, labelling it an attack on freedom of expression. Dan Kanemitsu, a manga translator, accused de Boer-Buquicchio of “mixing reality with fiction”, adding that there was a difference between abusive images featuring actual children and depictions of childlike characters in manga.

“There is no such thing as manga and anime child pornography,” he said. “Child pornography entails the involvement of children, and we must confront it for that reason. [De Boer-Buquicchio] meant sexualised depictions of childish looking characters in manga and anime. Many male and female artists in Japan draw characters in an art style that looks childish to western eyes. Therefore it is a rejection of an art style popular in Japan.”

Kanemitsu said he did not believe Japan’s resistance to expanding the ban to include manga would damage its international image. “I think many people will see the logic of protecting free speech, as long as no actual people are harmed,” he said.

But Japanese campaigners said they would increase pressure on the government to tighten the law. “Materials of children under 18 that were clearly created for the purpose of fulfilling sexual excitement should be regulated as child pornography,” Shihoko Fujiwara, head of the nonprofit organisation Lighthouse, told Agence France-Presse.

She pointed out that DVDs showing half-naked children were still widely available, adding: “We should teach children about sexual crimes at schools.