There are still too many bad

"There are still too many bad actors in search of these gucci shirts dangerous materials, and these dangerous materials are still vulnerable in too many places."

Terrorists only need a "handful" of nuclear material to kill hundreds of thousands of people, he warned.

"And that's not an exaggeration. That's the reality that we face."

Mr Obama said progress had been made since the first Nuclear Security Summit in Washington in 2010 but said more needed to be done.

"The security of the world depends on the actions that we take."

Ms Gillard will address leaders later on Tuesday, telling them the government has honoured commitments it made in 2010, including ratifying the Convention on Nuclear Terrorism and inviting nuclear inspectors into Sydney's Lucas Heights facility.

"They are small steps but that's because we do start from a high base," she told reporters before entering the summit on Tuesday.

Ms Gillard also will use her address to talk up Australia's nuclear security credentials more generally.

The not-for-profit Nuclear Threat Initiative has ranked gucci slippers Australia the top country in the world for nuclear security, with a score of 94 out of 100.

While the summit is focused on stamping out the threat of nuclear terrorism, North Korea's nuclear program looms large over the event.

Pyongyang's plan to launch a satellite using a long-range missile aimed at Australia's region - in breach of a UN ban and a US food aid deal - is dominating discussions on the summit's sidelines.

Ms Gillard repeated her call for North Korea to abandon the plan.

"Whilst the North Korean regime is a difficult regime for anyone to influence, countries with the best ability to influence it, like China, need to use their efforts to get North Korea to step back," she said.

The Fukushima nuclear disaster in Japan also will figure in the talks.
Mr Obama used the opening session of the Nuclear Security Summit in Seoul on Tuesday to urge more than 50 national leaders to stay committed to securing nuclear black gucci belt material so it never falls into terrorist hands.

"The threat remains," Mr Obama said.