By JOSHUA MITNICK

TEL AVIV—At least nine pro-Palestinian activists died when Israeli commandos boarded a ship headed to the blockaded Gaza Strip early Monday, plunging Israel into a diplomatic crisis that could obstruct action on the most pressing issues in the Mideast, from U.S.-backed peace talks to sanctions against Iran.

The incident drew harsh words from the Arab world and questions from foes and allies about why the operation to intercept the six-ship flotilla, for which Israel had weeks to prepare, turned deadly. The United Nations Security Council met in emergency session to consider condemning Israel’s actions.

The clash brought tensions with Turkey, one of Israel’s few friends in the Muslim world, to a boiling point. Thousands of Turks protested in Istanbul. And Ankara, which had publicly backed the flotilla, withdrew its ambassador to Israel.

Israel defended its actions, saying soldiers being lowered from a helicopter onto the deck of one of the ships, the Mavi Marmara—a Turkish-flagged ferry carrying 600 passengers—were beset by activists armed with metal poles, knives and guns.

Details of the clash remained sketchy, with information limited to briefings by the Israeli military, which cut off access to activists early Monday.

Even before the incident, Israel’s relations with even its most staunch ally, the U.S., were frayed. Prime Minister Benjamin Netanyahu, visiting Canada on Monday, canceled a visit slated for Tuesday to meet President Barack Obama in Washington—a meeting that Israeli and U.S. officials hoped would mend ties and give both sides an opportunity to show the strength of the alliance. An Israeli official said the prime minister “feels he has to be home to deal with this.”

Mr. Obama, in a phone call with Mr. Netanyahu, expressed regret at the loss of life and “expressed the importance of learning all the facts and circumstances around this morning’s tragic events as soon as possible,” a White House spokesman said.

“We…expect that the Israeli government will conduct a full and credible investigation,” said State Department Spokesman P.J. Crowley.

European allies were less circumspect. France and Spain quickly concluded the Israeli use of force was excessive. French President Nicolas Sarkozy said he “condemns a disproportionate use of force.” Spain called the Israeli actions “unacceptable.”

The European Union called for a full enquiry. The U.K. called for Israel to drop its blockade of Gaza.

The international censure could endanger some of the key foreign-policy aims of Mr. Netanyahu’s government, analysts said. Those include Israel’s stated top strategic priority: keeping Iran from obtaining a nuclear weapon.

Washington is pushing a fresh set of sanctions against Iran at the U.N., but the international reaction to the deaths aboard the Mavi Marmara, including the time needed to conduct a full investigation into the incident, could empower nations already resisting the effort.

The U.N. Security Council’s permanent members drafted proposed sanctions last month, a day after Turkey helped broker a deal with Iran intended to pre-empt sanctions. On Monday, Turkey again led the charge, at an emergency session of the Security Council to address the Mavi Marmar incident. Turkey drafted a resolution to present to the council that called Israel’s actions “an act of barbarism” and “aggression in the high seas,” and compared it to Somali piracy.

The incident could also have repercussions for U.S.-brokered peace talks between Israel and the Palestinians, a Washington priority. Any disruption to talks could put more strain on the U.S.-Israel relationship, which has been chilled in recent months by Israel’s insistence on construction plans in the West Bank and East Jerusalem.

Former Palestinian Prime Minister Ahmed Qurai called the deaths “a new crime against humanity,” according to a release on the Palestinian authority’s official news agency, WAFA. Despite that, Palestinian negotiator Saeb Erekat said that the indirect, U.S.-brokered talks on a peace deal with Israel would continue. “We are not going to cancel the meetings with the Americans,” he said.

But those talks involve the Palestinian Fatah faction, led by Mahmoud Abbas. Mr. Abbas had pulled out of talks in March, after Israel announced new construction in East Jerusalem, and he needed Arab backing to proceed. The broad Arab and Palestinian criticism of Monday’s incident will weaken that support.

Israel and Egypt began restricting the flow of goods into and out of Gaza in 2007, after Hamas seized control. Human-rights activists have long alleged Israel was keeping out crucial aid and basic materials, a claim Israel has denied. Israel and Egypt say they are preventing the smuggling of weapons and fighters.

Egyptian President Hosni Mubarak condemned “excessive use of force,” according to Egypt’s state news agency, but gave no indication Egypt would ease its Gaza blockade.

Israeli officials had said they were sensitive to the possible public-relations fallout from appearing too heavy-handed in dealing with the flotilla, which was organized, in part, by a controversial Turkish charity, the Humanitarian Relief Foundation, and several other groups, including the Cyprus-based Free Gaza Movement, which has sponsored past blockade-busting missions.

Last week, Israel said if the ships docked at an Israeli port first, it would allow the full shipment of humanitarian cargo to reach Gaza, after undergoing security checks. Israeli officials have for days warned that they wouldn’t let the flotilla reach port in Gaza. Organizers said they would ignore the warning an try to break the blockade.

The six-ship flotilla—carrying hundreds of pro-Palestinian activists from various countries, as well as 10,000 tons of construction material, medical equipment and school supplies, according to organizers—started heading to Gaza on Sunday afternoon, embarking from a rendezvous point off the coast of Cyprus, after several delays over the weekend.

After nightfall Sunday, three Israeli naval vessels left their base in Haifa, Israel, to intercept the ships, according to the Associated Press.

The Israeli military operation to board the vessels began at 4 a.m. Monday, according to Israel Defense Force spokesman Brig. Gen. Avi Benayahu, in an interview with Israeli Army Radio.

He said the boarding took place after the flotilla ignored calls to change course and head away from the Gaza coast. During the boarding, passengers on five of the ships resisted passively and peacefully, he said.

But on the Mavi Marmara, with about 600 passengers, the IDF encountered “difficult violence,” Gen. Benayahu said. Dozens of activists attacked Israeli soldiers with steel clubs and knives as the soldiers were being lowered onto the deck from helicopters, he said.

An account by an unidentified Israeli soldier indicated the activists’ response took troops by surprise. “We were in shock we were not prepared for that,” he said.

The army then used antiriot measures against the activists, he said. When the violence escalated, soldiers opened fire, he said. Activists used their own firearms to shoot at the Israelis, he said. It wasn’t clear from his comments who Israel believed shot first.

Israel’s military released video footage it said backed up its version of events. The grainy footage showed Israeli soldiers descending on ropes from helicopters. The deck is filled with soldiers and passengers fighting each other. Israel highlighted what it said was footage showing passengers beating soldiers with poles and chairs and another instance where a soldier appeared to be thrown off the deck.

By late Monday, Israel hadn’t allowed access to passengers aboard the ships, which state-controlled Israel Radio said had been escorted by Israeli naval vessels to the Israeli port of Ashdod. Israel Radio said passengers from the flotilla were being interviewed to determine whether they should be deported or detained.

Israel Radio said at least a dozen activists had been injured, as well as several Israeli military personnel.

Uzi Dayan, a former general who spent 17 years in the Israeli special forces, said helicopters are used to achieve an element of surprise, and because it is the only way to take over a ship that is sailing without endangering the entire vessel, though it leaves commandos outnumbered in the first minutes of the takeover.

Reuters

—Marc Champion in Istanbul, John W. Miller in Brussels, David Gauthier-Villars in Paris and Kathy Chen in Washington contributed to this article.

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