The UN secretary general, Ban Ki-moon, has called on Iran to impose a moratorium on executions after two juvenile offenders were hanged in the past two weeks.

Ban said he was “deeply saddened” by the executions and recalled that Tehran had signed two international conventions outlawing the death penalty for children under 18.

“The secretary general reaffirms the opposition of the United Nations to the imposition of the death penalty, and calls on the government of Iran to establish a moratorium on executions with a view to abolishing the death penalty,” said a statement from his spokesman.

Fatemeh Salbehi, a 23-year-old woman, was hanged in the province of Fars on 13 October after being found guilty for killing a man she had been forced to marry when she was just 16.

A week earlier Samad Zahabi was hanged in Kermanshah province for shooting a fellow shepherd when he was just 17.

UN human rights experts said Iranian authorities did not provide the required notice to Zahabi’s family or lawyer before the hanging.

Ban said he was “concerned that these two executions reflect a worrying trend in Iran”, where he said more than 700 people have reportedly been executed so far this year, including at least 40 publicly.

He said that was the highest total recorded in the past 12 years.

“The majority of executions were imposed for drug-related offences – crimes that do not meet the threshold of the ‘most serious crimes’ as required by international law,” he said.

More than 160 juvenile offenders are believed to be on death row in Iranian prisons, according to Amnesty International.

Iran – along with China, the United States, Iraq and Saudi Arabia – ranks among the world’s top executioners, according to Amnesty.