The Hamas leadership, along with other Islamic resistance groups, gave Israel a one-week ceasefire, during which it must withdraw all its forces from the Gaza Strip. In announcing the truce, Hamas said the Israelis' 22-day aggression had failed and bestowed the Jewish state no reason to reclaim victory.
Israel said it will not pull troops out at present, and will continue to attack opponents as necessary.
"After the failure of the Zionist aggression on Gaza in imposing its conditions on the resistance and our people – despite of more than 3 weeks of massacres, we announce holding fire in Gaza and we emphasize our stance and demands that Israel should withdraw from the Strip within a week," read Hamas' No. 2 leader Mousa Abu Marzouk, from a written statement broadcast on state-run Syrian TV. Representatives of other factions were sitting next to him.
"Israel also should open all border crossings to allow the entry of humanitarian aids and basic needs to our people in Gaza," he added. The Israeli air and land attack left large areas of the crowded territory in ruins and without basic services like potable water and electricity.
Today's announcement comes only hours after a unilateral Israeli ceasefire against Hamas and the deaths of more than 1,300 Palestinians. A missile and rocket exchange between Hamas militants and Israeli forces has immediately challenged the partial Gaza ceasefire early Sunday with Hamas firing five rockets on Israeli settlements.
"In light of this position, we are ready to respond with any efforts, especially the Egyptian, the Turkish, the Syrian and the Qataris', to reach a certain deal that would meet with our known demands so that to ultimately lift the blockade and open all crossings, including Rafah," Abu Marzouk said.
Before Sunday's announcement by Hamas, a missile and rocket exchange between militants and Israeli forces challenged the Gaza cease-fire.
Hamas has not said how many fighters it has lost, but the Israeli military puts the number at more than 500. On the Israeli side, 10 soldiers and three civilians have been killed, either in combat or by rocket fire.
Also on Sunday, in Damascus Syrian President Bashar Al-Assad told visiting U.N. Secretary General Ban Ki-moon that Israel's unilateral cease-fire in Gaza was not enough, saying the Jewish state should in addition completely withdraw its troops from the Strip and open the border crossings.
"President Assad underlined the necessity that the truce should go in parallel with Israel's pullout from the whole Gaza Strip, a total lift of the blockade and opening all the crossings to secure providing the basic needs to the Palestinian people," a Syrian source said after the lengthy meeting between Assad and the top U.N. official.
Assad told leaders meeting in the Gulf state of Qatar on Friday that the 2002 Arab peace initiative, which promises recognition of Israel in return for a withdrawal to the 1967 borders and a just settlement for Palestinians, had already died.
The Palestinian factions praised the resolutions of the Gaza summit in Doha. They received a huge political boost on Friday when its leaders attended the top-level gathering of the so-called "hardline" Arab countries (as well as Iran's Mahmoud Ahmadinejad, who is viscerally hostile to Israel) for the first time.
Khaled Meshal, the de facto Hamas leader, along with the leaders of Islamic Jihad and the Popular Front for the Liberation of Palestine, which are also fighting Israel in Gaza, flew to Doha from Damascus on the Emir of Qatar's private plane.
Israel accuses Syria and Iran of supporting and arming Hamas, an accusation they deny.
The meeting was boycotted by Egypt, Saudi Arabia, and Palestinian President Mahmoud Abbas.
Egypt will host an international summit on Sunday attended by several European leaders and U.N. Secretary General Ban Ki-Moon, to seek a lasting truce between Israel and Hamas in the Gaza Strip.
As he was heading to the Egyptian Red Sea city of Sharm el-Sheikh for a summit meeting on aid and reconstruction efforts for Gaza, Ban told reporters that Israel "needs to withdraw all of its troops as soon as possible."
Ban, who is on a tour of the region to secure a lasting cease-fire in Gaza, said that humanitarian access for the Gazans was his "top priority," after Israeli air and land attack left large areas of the crowded territory in ruins and without basic services like potable water and electricity.
By CBS News' George Baghdadi reporting from Damascus.