WASHINGTON — President Obama completed a first meeting of world leaders on combating nuclear terrorism with a list of specific commitments from dozens of nations to eliminate or lock down nuclear materials, in what he called a “bold and pragmatic” program to finish the task in the next four years.

But in a news conference after leading the conversation among 47 presidents, prime ministers and senior officials, Mr. Obama acknowledged that tough choices lay ahead on many of the far more politically volatile issues in stopping the proliferation of nuclear weapons.

He issued a specific warning to Iran, which was not represented at the conference, saying that after four years of failed efforts on sanctions, the penalties he was trying to win at the United Nations Security Council had to be significant enough to get the attention of the Iranian leadership.

Speaking to reporters, Mr. Obama said he had insisted to President Hu Jintao of China that in dealing with Iran: “Words have to mean something. There have to be some consequences.”