Obama made the comments before meeting for three hours with Israeli Prime Minister Binyamin Netanyahu amid worries over Iran and its nuclear program.
"As I've said repeatedly, the bond between our two countries is unbreakable" Obama said. "And as I've said to the prime minister in every single one of our meetings, the United States will always have Israel's back when it comes to Israel's security."
Obama, who talked up sanctions against Iran Sunday before the American-Israel Public Affairs Committee lobbying group, emphasized the two countries will continue to tighten pressure against Iran.
"My policy here is not going to be one of containment. My policy is prevention of Iran obtaining nuclear weapons," he said Monday. "And as I indicated [Sunday] in my speech when I say all options are on the table, I mean it."
Obama said that both he and Netanyahu prefer to resolve the issue diplomatically.
"We understand the costs of any military action," the president said.
In Sunday's speech to AIPAC, Obama warned of too much loose talk of war.
"Over the last few weeks, such talk has only benefited the Iranian government, by driving up the price of oil, which they depend on to fund their nuclear program," Obama said. "For the sake of Israel's security, America's security, and the peace and security of the world, now is not the time for bluster.
"Now is the time to let our increased pressure sink in, and to sustain the broad international coalition we have built. Now is the time to heed the timeless advice from Teddy Roosevelt -- speak softly; carry a big stick."
Netanyahu thanked Obama for the United States' continued support of Israel.
"Americans know that Israel and the United States share common values, that we defend common interests, that we face common enemies. Iran's leaders know that, too," he said before the meeting in the Oval Office.
"I believe that's why you appreciate, Mr. President, that Israel must reserve the right to defend itself. And after all, that's -- that's the very purpose of the Jewish state: to restore to the Jewish people control over our destiny. And that's why my supreme responsibility as prime minister of Israel is to ensure that Israel remains the master of its fate"
Netanyahu has said he wants Obama to take a tougher stand against Iran than the administration's current position -- specifically to be more explicit about the circumstances under which the United States would carry out a strike, The New York Times reported Monday.
In his AIPAC speech, Obama appeared to green-light any action Israel deems necessary.
"We all prefer to resolve this issue diplomatically. Having said that, Iran's leaders should have no doubt about the resolve of the United States, just as they should not doubt Israel's sovereign right to make its own decisions about what is required to meet its security needs," Obama said.
After the meeting, the White House issued a readout, saying the two leaders also discussed Middle East peace efforts and the situation in Syria.
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