There are up to 500,000 child soldiers around the world, Nobel laureate and child rights activist Kailash Satyarthi said Friday, terming it the worst form of child abuse.

Satyarthi, who is attending an International Labour Organization summit in Geneva, said global funding for education -- the best means to fight child labour and servitude -- had gone down dramatically over the past four years.

"There are between 400,000 and 500,000 child soldiers across the world but the actual numbers could be far higher because there are hidden militant groups kidnapping children and forcing them to use guns," he told reporters.

Satyarthi, whose decades-long campaign to end child labour in his native India and elsewhere won him the Nobel Peace prize last year, said "making children kill people is the worst thing you can do to a child."

"Look at what's happening in Syria, in Iraq, Nigeria and in Afghanistan," he said.

"When I read that a five-year-old child is given a gun to kill an opponent of a militia group in Iraq... and the child could not handle that gun, he was buried alive... That makes me angry. I think this should make you all angry," he told the summit Thursday.

Satyarthi said that global funding for education had plummeted in recent years "partly due to the financial crisis and partly due to donors' perceptions that education had made huge strides in developing countries when the case is not actually so."

All the world's children could access basic education if global annual funding for the sector was increased by $22 billion (19.6 billion euros), he said, adding that there were 58 million children currently out of school across the world.

"I have been advocating for the last 35 years that eradication of child labour and quality education are the two sides of the same coin. We cannot achieve the one without the other," he said.

"Unfortunately only one percent of the total humanitarian aid goes for education."

"An additional $22 billion annually can bring all children to school and that is just four-and-a-half days of global military expenditure.

"If we all decide that we are not going to fight, or strike... for four-and-a-half days we are going to bring all children to schools," he said.

According to the ILO there are 168 million children in child labour, with 150 million of them aged between five and 14.

Of them five million are thought to be trapped in slavery-like conditions.

Satyarthi said "millions of children in the world are living in danger."

"When we read in the newspaper that girls were being sold for sex slavery... for a price less than a cigarette pack in Syria, I feel ashamed," he said.