UNITED NATIONS - The UN Secretary-General Ban Ki-moon Thursday strongly condemned a bus attack occurred in northern Pakistan, killing at least 15 innocent passengers and causing multiple injuries.

In a statement released here by Ban's spokesperson, the UN chief strongly condemned the "appalling" attack, which reportedly targeted Shia Muslims.

"The Secretary-General expressed his outrage over such deliberate attacks on people due to their religious beliefs in Pakistan," said the statement. "He also extended his heartfelt condolences to the families of the victims and to the government and people of Pakistan." ?

At least 15 people were killed and several others injured when some unknown gunmen opened fire at three passenger vans in Pakistan's northern province of Gilgit-Baltistan on Thursday, according to local media reports in Pakistan.

No group have claimed responsibility for Thursday's attack yet.

This is the second incident of bus attacks in the area over the last six months. Earlier in February, 18 people were killed and five others injured in a passenger bus attack of a sectarian nature in Gilgit.