CAIRO — As the war in Gaza entered its 21st day with the quest for an elusive truce resuming in Cairo and Washington, a top exile leader of the Hamas militant group battling Israel’s onslaught rejected Israel’s terms for a cease-fire and urged Arab states to support its resistance.

In a speech broadcast across the Arab world and widely followed in the Middle East and elsewhere, Khaled Meshal, the senior leader in exile of Hamas, told an unusual Arab gathering in Doha, Qatar, that “I assure you: despite all the destruction in Gaza, we will not accept Israel’s conditions for a ceasefire.

“We tell our loved ones in Gaza, the aggression will soon perish on the rock of your steadfastness.”

Israel has long insisted that a cease-fire should be long-term and sustainable, preventing Hamas from firing rockets at Israel or re-arming.

But Mr. Meshal, who is based in Damascus, told the meeting in Doha that his organization would accept a cease-fire only if Israel withdrew its forces from Gaza, lifted its blockade of Gaza and re-opened border crossing points. Despite three weeks of fighting that has claimed around 1,100 Palestinian lives, he said, “resistance in Gaza has not been defeated. It has suffered harm but it has not been defeated.”

According to Egyptian officials who know details of the cease-fire negotiations, Hamas has been plagued by internal bickering, with the Syria-based leadership hewing to a tougher line on concessions from Israel, while the Gaza-based leadership has been more eager to halt the fighting and more flexible in negotiations.

The meeting in Doha followed a call by Qatar to hold an emergency Arab summit meeting on Gaza, but the proposal was opposed by regional powers such as Egypt and Saudi Arabia, reflecting broader and profound divisions in the Arab world.

At least two other Arab meetings have already debated or will debate the Gaza question. The Arab League says the Qatar meeting does not have the necessary quorum.

Mr. Meshal made the keynote address on Friday, securing a significant platform in his rivalry with Mahmoud Abbas, the president of the Palestinian authority based in the West Bank city of Ramallah. In his speech Mr. Meshal said settlements with Israel in the past had not brought solutions for Palestinians’ problems.

Earlier on Friday, the United Nations secretary general, Ban Ki-moon, met Mr. Abbas in Ramallah and urged Israel to consider a unilateral cease-fire, a step Israel immediately rejected.

Mr. Ban told a joint news conference with Mr. Abbas that a “unilateral declaration of a cease-fire would be necessary at this time” in order to halt the hostilities. “The fighting must stop,’” Mr. Ban said. “We have no time to lose.”

Mark Regev, the spokesman for Prime Minister Ehud Olmert, dismissed the notion, according to news reports.

“I don’t believe that there’s a logical expectation in the international community that Israel unilaterally cease-fire while Hamas would continue to target cities, trying to kill our people,” he said.

Gaza residents said Friday that the fighting seemed less intense than on the previous day, when Israeli forces shelled the headquarters of the United Nations Relief and Works Agency and other buildings in central Gaza. The strikes magnified condemnation of Israel, already heated because of the number of civilian deaths, and further strained fraught relations with the agency that provides aid to Palestinian refugees.

The strike, which Israel said was in response to enemy fire, came even as Israeli officials indicated some progress in the Egyptian-brokered cease-fire talks. The Israeli foreign minister, Tzipi Livni, left for the United States late on Thursday, seeking an internationally guaranteed mechanism to stop arms smuggling into Gaza through Egypt. At the same time, Amos Gilad, Israel’s top negotiator on Gaza, returned to Cairo to continue talks with Egyptian officials.

Israel tightened the military pressure on Hamas on Thursday, perhaps to push it closer to a cease-fire. On Friday, the Israeli military said it launched 40 air strikes overnight, hitting targets including Hamas gunmen and a mosque that Israel said was used for weapons storage.

The military also said at least three rockets were fired from Gaza into southern Israel on Friday — fewer than in recent days — without causing casualties.

Mr. Meshal’s comments in Doha followed the death of , a senior Hamas leader inside Gaza, Said Siam, who was killed along with his brother and his son when Israel bombed the house that they were in. Mr. Siam was the powerful interior minister of the Hamas-run government in Gaza and the overall chief of its security forces, a significant blow for Hamas days after Israel indicated that its military structure remained largely intact.

According to witnesses and hospital officials, four members of a family in a building next door to the Siams were also killed in the Israeli raid.

Thursday’s strike against the United Nations headquarters wounded three people, destroying with three shells a warehouse full of hundreds of tons of food and medicine, said John Ging, director of United Nations operations in the area.

The incident, a week after some 40 people were reported killed when an Israeli mortar shell struck near a United Nations school, underscored the difficult relations between Israel and the United Nations that stretch back to Israel’s founding.

But Israeli officials, including Prime Minister Ehud Olmert, on Thursday justified the attack on the refugee agency headquarters, saying that Hamas militants had fired at Israeli forces from within the compound.

United Nations officials vehemently denied the allegations. Mr. Ging, as he often has during the war, denounced Israel in extended televised interviews, and he questioned why Israeli liaison officers had never mentioned Hamas activity in the area, even though he said they were in constant contact.

Over many decades, Israel has questioned the neutrality of many of the organization’s branches and complained of institutional bias. While both sides have been making efforts in recent years to work more constructively together, Thursday’s incident served to pry open the divide.

Even though modern Israel came into existence months after a historic General Assembly voted in 1947 to partition Mandatory Palestine into separate Jewish and Arab states, the country’s first prime minister, David Ben-Gurion, famously wrote off the United Nations in the 1950s, using its Hebrew acronym to dismiss it as “UM, shmum.”

When it comes to the United Nations Relief and Works Agency, Israel is in a special bind. On one hand, Israeli officials say they recognize the vital role of the organization that provides food and other assistance to hundreds of thousands of Gaza’s poor.

On the other hand, the agency is often accused by critics in Israel and beyond of perpetuating the Palestinian refugee problem, being the only United Nations branch dedicated to a specific refugee population whose numbers, according to the agency’s criteria, constantly grow.

Christopher Gunness, a spokesman for the United Nations Relief and Works Agency, said that in a meeting with its representatives on Thursday, Israeli Army representatives “privately admitted” that the source of the militants’ fire was several hundred yards away from the compound.

“With every false allegation, the credibility of those accusing us is incrementally diminished,” Mr. Gunness said.

Citing agency representatives who were present during the attack, Mr. Gunness said three white phosphorus shells had hit the compound, causing fires that raged for hours, an allegation to which the Israeli military did not respond.

White phosphorus is a standard, legal weapon in armies, long used as a way to light up an area or to create a thick white smoke screen to obscure troop movements. While using it against civilians, or in an area where many civilians are likely to be affected, can be a violation of international law, Israel has denied using the substance improperly. On Wednesday, Hamas fired a phosphorus mortar shell into Israel, but no one was hurt.

In Israel, there is parallel outrage that the world is not vociferously protesting how Hamas uses civilians and civilian institutions in Gaza as a shield.

Yigal Palmor, an Israeli Foreign Ministry spokesman, went further, saying that most of the United Nations agency’s staff in Gaza were local Palestinians and alleging that a “large part are affiliated one way or another with Hamas.”

Three Israeli civilians have been killed in rocket attacks and 10 soldiers have died during the current campaign.