The British government on Monday called on Israel to open all crossings to allow unfettered access for aid to Gaza. Foreign Secretary William Hague meanwhile urged Israel to "act with restraint and in line with international obligations".

In a statement after the incident, White House spokesman Bill Burton said the United States "deeply regrets" the loss of life and injuries in the Israeli raid and that U.S. officials were "currently working to understand the circumstances surrounding this tragedy."

To deal with the aftermath, Israeli Prime Minister Benjamin Netanyahu has decided to cancel a trip to the United States, together with a scheduled meeting with U.S. President Barack Obama.

Russian Foreign Ministry noted in a statement that the incident was "a confirmation that Israel's blockade to the Gaza Strip must be lifted as soon as possible."

"Moscow condemns and expresses its deep concern over the issue, first of all for the loss of lives and injuries among the humanitarian convoy participants ... It is obvious that using arms against the civilians and stopping the vessels in open sea with no judicial ground is a gross violation of the international norms," the ministry said.

German Foreign Minister Guido Westerwelle has telephoned his Israeli counterpart Avigdor Lieberman and requested a comprehensive investigation, including demanding a quick clarification of the fate of five German nationals who were believed to be on board.

In a statement, French Foreign Minister Bernard Kouchner said that he was shocked and couldn't understand Israel's attack in the context that the initiative of the flotilla was known by international community already.

"Nothing can justify the use of such a violence that we condemn," Kouchner said, adding that the new conflict proved that current situation in Gaza was not stable and called urgent policy response to address the rising tension in that region.

Italian Foreign Minister Franco Frattini said Israel's attack was "absolutely serious" and he "absolutely deplored" the killing of civilians in the navy attack, but he also said the event "should not affect the progress of peace negotiations and the final solution for the Middle East."

Iran meanwhile called the regime in Tel Aviv a threat to global peace and security after the clash on Monday. Iranian President Mahmoud Ahmadinejad said that raid was indicative of Tel Aviv's weakness, and that "supporting the people of Palestine is equal to supporting peace and freedom of every nation."

The Spanish presidency of the European Union (EU) Monday condemned Israel for the attack. "Both the Spanish government and in its role as current president of the EU wishes to express its utmost condemnation of this action," Secretary of State for the EU Diego Lopez Garrido said.

Meanwhile, EU foreign policy chief Catherine Ashton also called for a timely enquiry into the deadly attacks on Monday, and for an "immediate, sustained and unconditional opening" of the crossing for the flow of humanitarian aid, commercial goods and persons to and from Gaza.

United Nations Secretary-General Ban Ki-moon said he was "shocked by reports of killing of people in boats carrying supply to Gaza" in international waters, and called for full investigations into the incident.

Robert Serry, UN special coordinator for the Middle East peace process, and Filippo Grandi, commissioner-general of the UN Relief and Works Agency also said in a joint statement that they "wish to make clear that such tragedies are entirely avoidable if Israel heeds the repeated calls of the international community to end its counterproductive and unacceptable blockade imposed on the Gaza Strip."

The UN Security Council is expected to meet on Monday afternoon for an emergency session to discuss the Israel's attack.

Meanwhile, the Cairo-based Arab League (AL) called on the international community to move as quickly as possible to stop Israel's actions which it described as terrorism and causing unrest in the region. The pan-Arab body has decided to hold an emergency meeting Tuesday on the incident.

"The Arab League Council will study all the repercussions and developments and take a stance that maintains the rights of the Palestinians and the legitimate Palestinian cause," AL's Deputy Secretary-General Ahmed bin Helli said Monday.