"The secretary-general condemns the killing of one of the five Iranian border guards who were abducted on 6 February in the country's southeast border region by a militant group," said a statement issued here Tuesday night by Ban's spokesman.
The secretary-general "expresses his solidarity with the government and people of Iran, who are confronted with this appalling act amid the annual Nowruz celebrations held to commemorate in peace the start of a new year," the statement said, referring to the Persian New Year.
"The secretary-general sends his condolences to the family of the slain guard," said the statement. "He calls for the perpetrators to be brought to justice."
Ban "hopes for the success of the ongoing efforts by the Government of Iran to achieve the release of those who remain captive," it said.
Iran's Sunni rebels kidnapped five border guards in southeastern Sistan and Baluchestan province of the country. The Jeish-al-Adl Sunni group, based in Pakistan, later claimed responsibility for the abduction by posting a photo of the kidnapped on its Twitter.
In October 2013, the armed clash between the Sunni rebel group and Iranian forces in a mountainous area near Saravan city bordering Pakistan left 17 guards killed and many others injured.
Last November, Jeish-al-Adl claimed responsibility for the assassination of a prosecutor in Zabol city in Sistan and Baluchestan.
Sistan and Baluchestan province, which is mostly populated by the Sunni Muslims, has been the scene of bloody clashes between the Sunni rebels and the Iranian security forces in the past years. The region is also located at the crossroad of the international drug smuggling route from Afghanistan and Pakistan to Europe.