Ukraine vowed to dispose of potentially weapons-grade uranium Monday in a dramatic kickoff to a 47-nation summit on bringing the world's nuclear ingredients under control -- and out of the hands of terrorists.

As the site of the 1986 Chernobyl power plant disaster, the worst civilian nuclear explosion, Ukraine knows first-hand the massive effects of radiation on an unprepared population.

Monday's pledge to get rid of all highly enriched uranium stores by 2012 came after President Barack Obama met Ukraine's new President Viktor Yanukovych on the sidelines of the summit.

The two-day conference, at a heavily guarded Washington convention center, is the largest global meeting hosted by a US leader since 1945.

Top powers were to discuss the increasingly diffuse nuclear threat, focusing especially on stocks of separated plutonium and enriched uranium which could be used by militant groups to manufacture crude, but devastating weapons.

Washington warned Al-Qaeda's interest in nuclear weapons was still "strong" and warned the risk of nuclear terrorism was "real," "serious" and "growing."

"Al-Qaeda has been engaged in the effort to acquire a nuclear weapon for over 15 years, and its interest remains strong today," said John Brennan, Obama's top anti-terrorism and homeland security advisor.

Obama was seeking concrete commitments -- like that made by Ukraine and a similar promise earlier by Chile -- that will secure loose materials in military and civilian stockpiles worldwide within four years.

"President Yanukovych announced Ukraine's decision to get rid of all of its stocks of highly-enriched uranium by the time of the next Nuclear Security Summit," the two leaders said in a joint statement

It added Kiev intended to remove substantial amounts of its stock this year.

Obama calls attempts by non-state groups to obtain nuclear devices "the biggest threat to US security, both short-term, medium-term and long-term."

"This is something that could change the security landscape of this country and around the world for years to come," Obama said Sunday.

Security was tight as the summit got underway.

Military police Humvees were parked outside the cavernous Washington Convention Center where leaders including Chinese President Hu Jintao and Russia's President Dmitry Medvedev were to meet.

A small, but noisy gathering of protestors demonstrated against Chinese rule in Tibet, while protestors from the Buddhist-inspired Falun Gong movement, which is banned in China, staged their silent, meditative exercise routines.

Although the conference was focused mainly on non-state threats, Obama's bilateral meeting with Hu was expected to address growing tension over Iran's nuclear program.

North Korea, which defied international pressure to produce a nuclear weapon, was also likely to loom over the summit. Neither the leaders of North Korea or Iran were attending.

Ali Asghar Soltanieh, Iran's envoy to the UN nuclear watchdog, the IAEA, spurned the summit, and accused Washington of being the "real" threat to global peace given its large nuclear arsenal.

"The outcome of the Washington conference is already known. Any decision taken at the meeting is not binding on those countries who are not represented at the conference," Soltanieh told ISNA news agency.

Obama was likely to use his meeting with Hu to argue for Beijing to allow its yuan currency to find a market level, as well as pressuring the Chinese to back UN sanctions against trade partner Iran.

The United States and its allies accuse Iran of seeking to build a nuclear weapon and want tough UN sanctions within weeks. Iran says its drive for nuclear technology is entirely civilian in character.

The US president was meeting privately with a raft of other foreign leaders on the sidelines of the conference.

Obama praised Ukraine for its decision to give up enriched uranium, saying this was "a historic step and a reaffirmation of Ukraine's leadership in nuclear security and nonproliferation."

Meanwhile, Obama and Jordan's King Abdullah II emerged from their bilateral meeting calling for Israeli-Palestinian proximity peace talks "as soon as possible," US officials said.

The US leader was to meet later with Armenian President Serzh Sarkisian and Malaysian Prime Minister Najib Razak.

One notable absentee will be Israeli Prime Minister Benjamin Netanyahu who dropped plans to attend, reportedly because of concern that Islamic states planned to press for Israel to open its own nuclear facilities to international inspection.

The conference is also a precursor to the United Nations Non-Proliferation Treaty review conference next month, seen as another important moment in heading off a future nuclear arms race.