WASHINGTON, April 13 (Xinhua) -- World leaders gathering in U.S. capital Washington on Tuesday called on states to secure vulnerable nuclear material in their respective countries in four years, in a bid to prevent terrorist and criminal organizations from obtaining such material.

In a Communique released as the nuclear security summit winds down, the 47 participating countries in the summit reaffirmed the "fundamental responsibility of states" in maintaining effective security of all nuclear materials, including those used in nuclear weapons and nuclear facilities under their control.

The leaders emphasized "the importance of robust national legislative and regulatory framework for nuclear security," in preventing non-state actors from obtaining the information or technology "required to use such material for malicious purposes."

According to the Communique, the nuclear materials in focus are highly enriched uranium and separated plutonium, weapons grade material that are used to make nuclear bombs. Leaders called for " special precaution" in handling those materials and agreed to promote measures to secure, account for and consolidate them.

The leaders mentioned converting reactors from using highly enriched to lowly enriched uranium fuel as one possibility in dealing with the problem.

The nuclear security summit was aimed specifically at dealing with the threat of nuclear terrorism, which the Communique recognizes as "one of the most challenging threats to international security."

U.S. President Barack Obama speaks at a news conference after the two-day Nuclear Security Summit closed in Washington D.C., capital of the United States, April 13, 2010.