"A truce is not on the table of negotiations," Khalid Mishaal told Egyptian television in an interview on Thursday.
Palestinian groups gathered in the Egyptian capital have opposed the concept of a unilateral cooling down period, preferring a fully-fledged truce, for which they would request something in return from Israel.
Truce not possible
Mishaal said Tel Aviv had not met the basic demands of resistance organisations, arguing that a truce deal was not possible.
"Israel has been flip-flopping on the issue of the prisoner release, the evacuation of cities and a halt to the construction of the separation wall, as well as other demands," he said.
"This position does not offer circumstances conducive to a truce," Mishaal said. "We have therefore decided to extend the cooling down period in order to give the other side a chance to deliver on its commitments."
According to Palestinian officials taking part in the talks, a final statement was expected to be issued later on Thursday.