WASHINGTON — President Obama strongly defended last week’s preliminary agreement with Iran as a “once in a lifetime opportunity” to curb the spread of nuclear weapons in a dangerous region while reassuring critics that he would keep all options available if Tehran ultimately cheated.

As he sought in an interview with The New York Times to sell the tentative deal to skeptics accusing him of giving away too much, Mr. Obama emphasized to Israel that “we’ve got their backs” in the face of Iranian hostility. And he suggested that he could accept some sort of vote in Congress if it did not block his ability to carry out the agreement.

“This is our best bet by far to make sure Iran doesn’t get a nuclear weapon,” Mr. Obama said in an interview with Thomas L. Friedman, an Op-Ed columnist for The Times, published on Sunday. “What we will be doing even as we enter into this deal is sending a very clear message to the Iranians and to the entire region that if anybody messes with Israel, America will be there.”

In the interview, held Saturday, Mr. Obama provided new details about how international inspectors would try to access suspected covert nuclear sites and about the sequence that would lead to sanctions being lifted. Both were major issues in the last days of negotiations in Switzerland, and Mr. Obama’s descriptions differed in key respects from Iran’s interpretations.