Thousands of Hamas supporters have attended a rally in Gaza to honour assassinated commander Mahmoud al-Mabhouh.
The group has blamed his death last month in a luxury hotel in Dubai on an Israeli hit squad.
"The decision to avenge the martyr Mahmoud al-Mabhouh has been taken, and it will be equal to the crime," Abu Obeida, a spokesman for Hamas's armed wing, told the crowd at Wednesday evening's rally in Gaza.
"All you killers can do now is wait," he said.
"We are the ones who will decide the tools suitable to carry out our promise. We will not tell you how or where or when, but only to prepare to receive the hellfire of our anger."