Try as he might to focus Asian and Pacific leaders on forging new economic partnerships during a regional summit here, President Obama has spent as much time in private meetings with his counterparts discussing another pressing concern: national security.

Among the key issues Obama raised in formal bilateral discussions were containing nuclear proliferation in Iran and North Korea, establishing clear maritime rights in the South China Sea and working out the future of U.S. military forces in Japan, senior White House officials said.

While several Republican presidential candidates criticized the president’s policy on containing Iran’s nuclear weapons program during a debate Saturday night, Obama was pressing the leaders of China and Russia on the importance of presenting a united front that “makes clear to Iran that this is unacceptable,” an Obama adviser said.

The United States has built an international coalition whose sanctions have left Iran more politically isolated than ever and ground its economy “to a halt,” said Ben Rhodes, a deputy national security adviser for strategic communications.

At a news conference Sunday, Obama said that the sanctions “have enormous bite and enormous scope” even thought the International Atomic Energy Agency reported last week that Iran may be proceeding with its nuclear weapons program.

“The question is: Are there additional measures we can take?” Obama said. “We’re going to explore every avenue to see if we can solve this issue diplomatically. We are not taking any option off the table. Iran with a nuclear weapon would pose a security threat not only to the region but to the United States.”

At the Asia Pacific Economic Cooperation summit this weekend, Obama and other leaders tried to keep security issues on the sidelines of the formal economic growth agenda. But security challenges around the world have been impossible to ignore here.

At the news conference, the president was asked about criticism from his Republican presidential rivals who said that he was wrong to ban the practice of waterboarding foreign nationals suspected of terrorism.

“They’re wrong. Waterboarding is torture,” Obama declared. “It’s contrary to America’s traditions. It’s contrary to our ideals. That’s not who we are. That’s not how we operate it. We don’t need it to prosecute the war on terrorism and we did the right thing by ending that practice. If we want to lead around the world, part of leadership is setting a good example. . . . That’s not something we do. Period.”

Obama has tried to cast the United States as reasserting itself as a leader in the Asia Pacific after years of focusing on wars in the Middle East. But that could mean being drawn into more sticky situations, such as the fight between several Southeast Asian countries and China over maritime rights to the South China Sea, a crucial commercial shipping lane that is thought to contain valuable oil and minerals.

Officials said that the United States does $1.2 billion in annual trade through the South China Sea.

But China has made a claim to a large portion of the sea and backed it up with military confrontations aimed at cowing smaller rivals.

Adm. Robert F. Willard, head of the U.S. Pacific Command in Hawaii, said that the United States and its allies in the region have “expressed concern” about China’s recent assertiveness.

A serious confrontation is “exactly what the . . . United States military and the regional militaries are intending to prevent,” he said.

Obama is scheduled to arrive in Australia on Wednesday to tour a military facility in Darwin and announce a small buildup of U.S. forces in the region: a new partnership that will allow U.S. Marines to conduct training and exercises at Australian military bases.

The move is aimed in part at placing U.S. military resources outside the range of China’s ballistic missiles and repositioning troops under an ongoing “force posture review” that includes changes to the level of American forces in Japan.

The United States aims to move thousands of troops off the crowded island of Okinawa and put them in Guam.

Obama and Japanese Prime Minister Yoshihiko Noda discussed the matter in their bilateral meeting, said Daniel Russel, senior director for Asia at the National Security Council. He declined to be specific but said the steps Noda described were “modest.”