U.S. President Barack Obama on Monday evening welcomed world leaders to a working dinner in Washington, kicking off a summit aimed at preventing terrorists from obtaining nuclear weapons.

After a ceremonial welcome at the Washington Convention Center that saw leaders arrive separately and be greeted by Obama, the working dinner went underway, and leaders are likely to focus their discussion on the threat of nuclear terrorism during the dinner.

Before the meeting officially went underway, Obama conducted a series of bilateral meetings with leaders participating in the summit, and he secured some solid support.

In the meeting with Obama, Ukrainian President Victor Yanukovich pledged to get rid of the country's stockpile of highly enriched uranium by 2012, and the United States has agreed to provide technical and financial assistance in that endeavor.

The United States has invited 46 countries to participate in the summit, many of them sending their heads of state or government. The summit is aimed at securing loose nuclear materials around the world in four years, and a joint communique and a specific work plan is likely to be released after Tuesday's two plenary sessions.

The communique is likely to iterate the participating countries' commitment to the goal of securing nuclear materials in four years, and the work plan will probably line out specific steps as to how the broad goals and commitments of the communique will be implemented.

According to White House officials, Tuesday's plenary session in the morning will be focused on national actions countries can take to secure nuclear material under their control and to deal with the risk of nuclear smuggling within their territory.

They have voiced expectation that some countries will announce steps they are taking to either remove the presence of some nuclear materials on their territory or to consolidate them to protect them better, such as the pledge to remove weapons grade uranium made by Ukraine.

After the morning session, another plenary session will follow in the afternoon. Since the morning session will be focused on actions countries can take on a national level, the afternoon session will be focused on international measures that countries can take to strengthen the international system for dealing with nuclear security.

Obama was optimistic in securing the endorsement of the world leaders for his vision.

He said the level of world leader participation in the summit is "an indication of how deeply concerned everybody should be with the possibilities of nuclear traffic."

"I think at the end of this we're going to see some very specific, concrete actions that each nation is taking that will make the world a little bit safer," he told reporters before the welcome ceremony.