A top Israeli envoy delivered his country's stance on a cease-fire agreement in Gaza to Egyptian mediators trying to seal a truce on Thursday. Iranian President Mahmoud Ahmadinejad president said the fighting showed Israel's continued existence in the region is "not feasible."

The development came as the U.N. secretary-general pressed Israel, and Gulf leaders gathered in Saudi Arabia to discuss the conflict.

The diplomatic push gained momentum despite competing agendas among Arab and Islamic governments, who are openly disagreeing about how to resolve — or even discuss — the conflict between Israel and Gaza's Hamas rulers.

Meanwhile, Israeli troops pushed deeper into the densely populated Gaza City on the 20th day of the offensive to rout out Hamas militants. Israeli tanks shelled the crowded downtown, sending terrified residents fleeing for cover.

Witnesses and U.N. officials said Israeli shells struck the United Nations headquarters building that serves as a shelter for hundreds of people, setting it ablaze.

The Israeli push ratcheted up pressure on Hamas to accept a proposed cease-fire. It also came as U.N. chief Ban Ki-moon was in Israel trying to promote a cease-fire.

Mark Regev said Israel wants a total end to Hamas' rocket launches into Israel, and an arms embargo on Gaza's militant rulers.

"There is momentum in these discussions," Regev told AP Television News. "We are hopeful that a deal will be based on a total cessation of Hamas fire into Israel and an arms embargo to prevent Hamas from rearming is close and attainable."

Regev said the Israeli envoy — Amos Gilad, who flew to Egypt on a private plane — will discuss the "parameters of the end game." He will not be meeting Hamas envoys who are also in town.

Gaza-based Hamas official Ghazi Hamad said the deeper incursion reflected pressure on his group.

"I think Israel is seeking in the last moments to escalate the military operation to pressure the parties," Hamad told The Associated Press. "I don't think this will change the issues on the table."

Hamad said his group has offered amendments to Egypt's original peace proposal, and he expected the Egyptians will convey them to the Israelis. "Consultations are continuing," he said.

Hamas' deputy chief Moussa Abou Marzouk, who is based in Damascus, took a hard line on a ceasefire, telling The Associated Press that the group would not abandon its demand that that Israel withdraw its troops from Gaza. "This is our main demand, along with the reopening of the border crossings," he said.

The Egyptian proposal calls for a 10-day cease-fire but would delay any Israeli withdrawal until an arrangement is negotiated at border crossings to ensure weapons cannot be brought in. It was not clear if Abou Marzouk's comments were a rejection of that position.

A long term truce is to be discussed later, Marzouk said, adding he expected "clear answers" from the Israelis through the Egyptians on Thursday.

In Tehran, Ahmadinejad called on Saudi Arabia's King Abdullah to speak out over "the massacre of your children in Gaza," the official Iranian news agency reported.

Saudi Arabia is overwhelmingly Sunni, as are the Palestinians.

Ahmadinejad said a firm Saudi stand would dash hopes of those who want to divide Islamic countries.

At a news conference, Ahmadinejad said the fighting in Gaza has been "a great lesson for all," saying it shows "the absolute defeat and desperation of this (Israeli) regime."

He says that "even for the supporters of the occupying regime and its leaders, it has become clear that the continuation of the Zionist regime's life in the region is not feasible."

He urged Arab states to pressure Israel's Western backers to stop the fighting and to cut all ties with Israel, and also dismissed allegations Iran is urging Hamas to reject Egyptian truce efforts.

Israel says it launched the offensive Dec. 27 to stop rocket fire against southern Israeli towns by Hamas, which has controlled the Gaza Strip since 2007. Iran is Hamas' main backer, providing political and financial support. Iran denies sending Hamas weapons.

Meanwhile, an emergency summit of the six Gulf Cooperation Council countries, called by Saudi Arabia to discuss Gaza, is to take place in Riyadh later Thursday.

But a separate summit by Arab League heads of state called by Qatar for Friday in Doha was in doubt as Qatar couldn't get a two-thirds majority of the organization to attend.

Egypt and Saudi Arabia are against the Doha summit, believing it could scuttle Egyptian efforts to broker a truce between Israel and Gaza's Hamas rulers.

Gaza medical officials say 1,100 Palestinians have been killed since Israel's offensive started Dec. 27.