"The Israeli Army is no longer in any condition to achieve its goals," Ramadan Shalah was quoted as saying by the Palestinian news agency Maan.
He also claimed that the Israeli military command was "now thinking of how to leave Gaza without admitting defeat."
The Islamic Jihad Movement in Palestine is listed as a terrorist group by Western states and Israel. It has also clashed with Hamas, with controls the Gaza Strip, on several occasions.
Shalah also said that concluding a ceasefire agreement between Israel and Gaza would be possible only after three conditions were met: "a halt to Israel's aggression, a complete withdrawal of Israeli troops from Gaza, and the removal of blockades from the enclave."
Israeli Prime Minister Ehud Olmert has stated that Israel's main aims in the conflict are to stop homemade rockets and mortars from being fired by Palestinian militants from Gaza into Israel's southern territories, and halt the smuggling of weaponry into the enclave.
Last week, Olmert dismissed UN Security Council's Resolution 1860, which called for an immediate ceasefire in the Gaza Strip.
According to Israel's Haaretz daily, Olmert is now planning to raise the issue of continuing military operations in the Gaza Strip with the security cabinet in order to get majority support from key ministers and security chiefs.
The Palestinian Health Ministry has announced that over 900 Palestinians, including 257 children and 68 women, have been killed and another 4,000 wounded in the Israeli assault on Gaza, which started with coordinated airstrikes on December 27.
Israel's casualties are estimated at 13 people, including 10 military personnel.