New York  - United Nations Secretary General Ban Ki-moon on Thursday condemned the Israeli Defense Force for firing on a relief convoy and killing two UN workers while injuring another in the Gaza Strip.

The deaths brought to four the number of UN relief workers killed in the Israel-Hamas conflict, which entered a 13th day on Thursday.

"The United Nations is in close touch with the Israeli authorities about full investigation of this and other incidents and about the need for urgent measures to avoid them in the future," said UN spokeswoman Michelle Montas.

"The secretary general calls once again for an immediate ceasefire in order to facilitate full and unhindered humanitarian access and to allow aid workers to work in safety to reach persons in need," she said. "The inability of the UN to provide assistance in this worsening humanitarian crisis in unacceptable."

UNRWA, the UN refugee organization in the region, on Thursday suspended its operations in Gaza following the deaths of the two workers. (dpa)