U.S. President Barack Obama says nuclear terrorism is still one of the world's greatest security threats, despite success in cutting U.S. and Russian nuclear stockpiles.

Mr. Obama said Monday that terrorists and criminal gangs are doing everything to get their hands on nuclear, biological and chemical materials. He said these materials are being stored without enough protection.

He said the world cannot allow the 21st century to be darkened by the worst weapons of the 20th century.

Mr. Obama also called for an updated agreement with Russia, which has announced it will not renew its participation in the Nunn-Lugar pact — the 20-year-old law that eliminates nuclear missiles and chemical weapon stockpiles in the former Soviet Union.

President Obama said that thanks to Nunn-Lugar, thousands of missiles, bombers and submarines have been destroyed or deactivated.

The Nunn-Lugar Cooperative Threat Reduction program was the brainchild of former Democratic Senator Sam Nunn and retiring Republican Senator Richard Lugar.