The United States will seek firm commitments from world leaders next week to clamp down on the trafficking of weapons-grade nuclear materials, officials said Wednesday.

A two-day security summit of 47 nations kicking off Monday in Washington aims to raise international awareness about the threat of "nuclear terrorism," said James Miller, a senior defense policy official.

Countries would be encouraged "to accept responsibility for taking practical steps to reduce the likelihood that terrorists will get their hands on nuclear materials and be able to build a bomb," said Miller, the principal deputy under secretary of defense for policy.

"So they'll be talking about a variety of nuclear security measures that each of them can take within their own countries to prevent theft or seizure of nuclear materials and prevent transit, smuggling of nuclear materials through their territories."

Robert Einhorn, the State Department's special advisor for nonproliferation and arms control, told a briefing that officials were already hard at work on a joint statement and a specific work plan for the summit.

The United States is participating leaders to endorse a global crackdown on the illicit trade of nuclear materials, The Wall Street Journal reported.

The newspaper cited a copy of a draft US communique calling for tougher prosecution of traffickers, better accounting of weapons-grade nuclear materials and greater international collaboration.

The international community must "effectively prevent and respond to incidents of illicit nuclear trafficking," the draft document says, according to the Journal.

President Barack Obama has already said he hopes the summit will produce a commitment to secure all loose nuclear material across the globe within four years.

The United States unveiled new limits on the nation's nuclear arsenal Tuesday, saying it would only use atomic weapons in "extreme circumstances" and would not attack non-nuclear states.

In a policy shift, the United States said for the first time that countries without atomic weapons that complied with non-proliferation treaty obligations need not fear a US nuclear attack.

But Obama warned exceptions could be made for "outliers" such as Iran and North Korea, both accused of flouting UN resolutions.

The Nuclear Posture Review also described "nuclear terrorism" as an immediate and extreme threat, with efforts to prevent the spread of atomic weapons given top priority.

Obama is due to sign a treaty with Russia on Thursday to slash stockpiles of long-range nuclear warheads by a third. The president has also committed the United States to a series of nuclear arms cuts in a bid to bolster efforts to prevent the spread of nuclear weapons.

He is scheduled to meet Chinese President Hu Jintao, German Chancellor Angela Merkel, Prime Minister Manmohan Singh of India and Jordan's King Abdullah II on the sidelines of next week's summit.

But Obama has no plans to hold talks with Israeli Prime Minister Benjamin Netanyahu at the summit after the Israeli leader's acrimonious visit to the US capital last month.

There will also be talks with President Jacob Zuma of South Africa, Pakistani Prime Minister Yousuf Raza Gilani, President Serzh Sarkisian of Armenia, Malaysian Premier Najib Razak and Kazakh President Nursultan Nazarbayev.