Washington (CNN) -- President Obama said Tuesday that the 47-nation nuclear security summit he convened raised global awareness of the threat of nuclear terrorism and yielded commitments to better secure nuclear arsenals and materials.

At a news conference wrapping up the two-day summit, Obama cited steps taken by countries including Russia and other former Soviet states to eliminate some of the vulnerable vestiges of nuclear stockpiles from the Cold War era.

However, Obama was unable to declare a breakthrough with China and Russia on imposing tougher sanctions against Iran over its nuclear ambitions.

He acknowledged that the final communique unanimously adopted by the summit participants was non-binding but said the steps announced this week showed that nations were seriously committed to the issue of keeping nuclear materials out of the hands of terrorists "who would surely use them."

"We've made real progress in building a safer world," Obama said.

During the summit, the United States and Russia signed an update to a 2000 agreement intended to eliminate weapons-grade plutonium from their military programs.

Secretary of State Hillary Rodham Clinton said the agreement commits each country to "irreversibly and transparently" dispose of at least 34 metric tons of weapons-grade plutonium, enough for 17,000 nuclear weapons.

In addition, Russian President Dmitry Medvedev confirmed plans to close a plutonium production reactor in the Siberian city of Zheleznogorsk, the White House said. Obama called it an "important step forward."

Earlier, the United States, Canada and Mexico agreed to work to convert the fuel at Mexico's nuclear research reactor to a lower grade of uranium unsuitable for nuclear weapons, the White House announced.

The program to be overseen by the International Atomic Energy Agency would eliminate all highly enriched uranium in Mexico, according to the White House statement.

On Monday, Ukraine announced that it would get rid of its highly enriched uranium within the next two years. In addition, Canada said it would send highly enriched uranium from an Ontario nuclear power plant to the United States for safekeeping.

The announcements followed the removal last week of highly enriched uranium, which is used in making nuclear weapons, from Chile, the U.S. National Nuclear Security Administration said on its Web site.

A summit document issued Tuesday also noted other steps taken by participating nations, including a comprehensive nuclear law passed in March by Egypt to strengthen nuclear security, as well as funding contributions by several countries to nuclear regulatory agencies and programs.

Obama said the summit's final statement acknowledges the urgency and seriousness of the threat of nuclear terrorism and sets a goal for securing all the world's vulnerable nuclear materials in the next four years.

The communique calls for strengthening international institutions such as the United Nations and the IAEA, and recognizes the "fundamental responsibility" of nations to meet their international obligations regarding the Nuclear Non-Proliferation Treaty and other agreements, Obama said. He also noted that the communique stresses that progress on such issues requires international cooperation.

"We've got world leaders who have just announced that in fact this is a commitment they are making," Obama said. "I believe they take their commitments very seriously."

In response, a statement by Republican Sen. Jon Kyl of Arizona said the summit failed to achieve progress on what he called the most pressing issue: taking concerted international steps against Iran's efforts to develop a nuclear weapons program.

"The summit's purported accomplishment is a non-binding communique that largely restates current policy and makes no meaningful progress in dealing with nuclear terrorism threats or the ticking clock represented by Iran's nuclear weapons program," Kyl's statement said.

Obama recognized that China and other nations have reservations about the economic impact of tougher sanctions on Iran, an oil-producing state and trade partner for many, but said he would continue to push for speedy agreement of a new U.N. Security Council resolution on the issue.

Obama pointed to Ukrainian President Viktor Yanukovych's decision to get rid of his country's highly enriched uranium, a component in nuclear weapons, as an example of the serious intentions of participants. When asked about the security of Pakistan's nuclear arsenal in a nation known for political instability and terrorist links, Obama said he was confident that the weapons and materials were properly safeguarded.

At the same time, he said, all nations including the United States can do more to protect their nuclear weapons and materials.

After a working dinner Monday night, the leaders reconvened for a group photo Tuesday before entering the meeting hall for the day-long plenary session chaired by Obama around a large circular table.

Security concerns for the meeting locked down part of the city's downtown core, with streets closed and temporary fencing erected around the convention center where it was held. Parked military vehicles and city hauling trucks blocked access at some points to all but official vehicles.

Tuesday's plenary began with a moment of silence in honor of the victims of the plane crash over the weekend that killed Poland's president, first lady, military leaders and other top officials.

Obama also announced that South Korea has agreed to host the next nuclear security summit, which will take place in 2012.

Referring to the changing global situation regarding nuclear weapons, Obama said it was "a cruel irony of history" that the risk of nuclear confrontation between nations has decreased while the risk of nuclear terrorism has risen.

He noted that terrorists needed only "plutonium about the size of an apple" for a weapon capable of causing widespread death and destruction.

Obama convened the summit as part of a broader strategy to decrease the threat of nuclear weapons and technology from reaching terrorists. It was considered an unprecedented effort to rally global action on securing vulnerable nuclear materials.

The summit came less than a week after Obama signed a treaty with Russia to reduce the nuclear stockpiles of both nations, and his administration issued a revised U.S. nuclear arms strategy intended to reinforce the nation's nuclear deterrent while isolating terrorists and rogue states that fail to comply with international regulations.

It all fits together as an ambitious effort to mobilize a unified global effort against nuclear proliferation, U.S. officials say.

Of particular concern are older nuclear weapons and materials that aren't safeguarded as well as they should be, particularly in Russia and other states of the former Soviet Union, according to U.S. officials.

Participating in the summit were Algeria, Argentina, Armenia, Australia, Belgium, Brazil, Canada, Chile, China, the Czech Republic, Egypt, Finland, France, Georgia, Germany, India, Indonesia, Israel, Italy, Japan, Jordan, Kazakhstan, Malaysia, Mexico, Morocco, Netherlands, New Zealand, Nigeria, Norway, Pakistan, Philippines, Poland, the Republic of Korea, the Russian Federation, Saudi Arabia, Singapore, Switzerland, South Africa, Spain, Sweden, Thailand, Turkey, United Arab Emirates, the United Kingdom, Ukraine and Vietnam.

In addition, the United Nations, the European Union and the International Atomic Energy Agency were represented.