JERUSALEM — With the United States and other nations extending an increasingly warm welcome to the new president of Iran at the United Nations this week in New York, Israel finds itself in a bind: eager to unmask what it sees as an empty charm offensive, yet at risk of being seen as a spoiler unwilling to consider the possibility of change in Tehran’s nuclear policy.

Prime Minister Benjamin Netanyahu ordered Israel’s delegation to boycott Tuesday’s appearance by Iran’s new president, Hassan Rouhani, at the United Nations General Assembly and later denounced Mr. Rouhani’s address there as “a cynical speech that was full of hypocrisy.”

In a statement released after 2 a.m. Wednesday in Israel, Mr. Netanyahu said Iran’s strategy was “to talk and play for time in order to advance its ability to achieve nuclear weapons.”

Earlier Tuesday Mr. Netanyahu expressed appreciation for President Obama’s statement in his United Nations address that “Iran’s conciliatory words will have to be matched by action that is transparent and verifiable.” Mr. Netanyahu said, “Israel would welcome a genuine diplomatic solution that truly dismantles Iran’s capacity to develop nuclear weapons.” But he said that so far Iran had offered only “cosmetic concessions.”