Washington (CNN) -- For years, Turkey has been Israel's strongest predominantly Muslim ally in a region where the Israelis have few other friends.
Now Israel's military raid on a flotilla of ships backed by Turkey carrying relief supplies for blockaded Gaza threatens to unravel the relationship, presenting a major diplomatic headache for the United States.
Both countries are strategic U.S. allies in the Middle East -- Israel as a major partner in the Muslim-dominated region and Turkey as a NATO member.
However, Israel's storming of the six-ship flotilla -- resulting in the deaths of at least nine Palestinian activists while leaving seven Israeli soldiers wounded -- forces the Obama administration to seek a middle-ground response that offends neither.
Evidence of the crumbling Israeli-Turkey relations was everywhere Monday, from the chamber of the U.N. Security Council in New York to the streets of both countries.
Demonstrators in Istanbul, Turkey's largest city, tried to storm the high-rise building that contains the Israeli Consulate, some shouting "Zionist dogs!"
In Tel Aviv, Israel, demonstrators gathered in front of the Turkish Embassy carrying signs that said "Provocation is not a way for peace" and "Erdogan-Islamo-fascist" in reference to Turkey's president, according to Ynet, Israel's largest news website.
At an emergency U.N. Security Council meeting called by Turkey, Foreign Minister Ahmet Davutoglu accused Israel of piracy and "murder conducted by a state."
"No state is above the law," Davutoglu said, adding that "Israel must be held accountable for its crimes."
Turkey also recalled its Israeli ambassador, canceled three planned joint military exercises with Israel and called for Israel to apologize.
Israeli officials, while avoiding much direct criticism of Turkey, made clear that they believed the flotilla was a thinly disguised effort to break the Israeli blockade of Gaza.
"If indeed it were a humanitarian mission, it would have accepted, weeks ago, during the planning stages, the offer by the Israeli authorities to transfer the aid, through to the port of Ashdod, to Gaza through the existing overland crossing, in accordance with established procedures," said Daniel Carmon, Israel's deputy permanent U.N. representative.
Carmon also noted that Palestinian activists included members of the Turkish Foundation for Human Rights and Freedom and Humanitarian Relief, known by its Turkish acronym IHH, which he said had links to terrorist organizations including al Qaeda.
Further evidence of the deepness of the rift was the report that 15,000 Israelis canceled vacation plans for this year on the Turkish coast, one of the few places in the region where they previously were welcome.
The initial U.S. response expressed sorrow for the loss of life while calling for a full investigation, but stopping short of making any public judgment.
Traditional U.S. support for Israel surfaced in the Security Council meeting, where Alejandro Wolff, the deputy permanent U.S. representative, said the relief aid bound for Gaza should have been transported through accepted international mechanisms set up because of the Israeli blockade.
"These non-provocative and non-confrontational mechanisms should be the ones used for the benefit of all those in Gaza," Wolff said. "Direct delivery by sea is neither appropriate nor responsible, and certainly not effective, under the circumstances."
It was unclear whether the United States would take sides any further, such as backing Turkey's call for Israel to apologize for its raid on the flotilla.
Namik Tan, the Turkish ambassador to the United States, called the U.S. response so far "sort of weak." Asked what Turkey wants the United States to do on any potential statement or censure from the Security Council, the ambassador said the question was too "hypothetical."
He added, however, "Israel should not get away with this."
After years of cooperative relations with Israel that has built a strong trade relationship and military ties, Turkey protested strongly against the Israeli military's offensive in Gaza in 2008.
Now Turkey is calling for Israel to identify and send home the dead from the flotilla assault, and release all detained activists as well as the seized ships.
So far, Israel has shown no inclination to do so.
CNN's Ivan Watson, Ben Wedeman, Tom Watkins contributed to this report.