Ukrainian security forces seized nearly 375 pounds of a radioactive material seen as a likely ingredient for a "dirty bomb" (search), authorities said Thursday.

In a joint action, Ukraine's police and state security agents seized two containers of cesium-137 (search) and arrested three men from the southern city of Simferopol on the Crimean peninsula, police spokesman Yuriy Kondratyev told The Associated Press. An unspecified number of people also were detained throughout Ukraine.

Cesium-137 is considered a likely ingredient for a so-called "dirty bomb," in which conventional explosives are combined with radioactive material.

Cesium-137, a highly radioactive material, is used in soil-testing gauges in construction and is found in photoelectric batteries and vacuum valves. It explodes if it comes into contact with water, and exposure to it can cause blood diseases, sterility and birth defects.

Police and state security agents acted on a tip-off that two buyers from the Ukrainian capital, Kiev, were ready to purchase cesium at an estimated price of $60,000 per container, Kondratyev said. Each container seized weighed more than 187 pounds, he said.

Police declined to detail where the cesium was from or what roles the three suspects played in the case.

Western countries and the U.N.'s International Atomic Energy Agency (search) have repeatedly warned that former Soviet republics, including Ukraine, have become a traffickers' marketplace for radioactive materials.

Earlier this year, Ukrainian authorities arrested a man trying to take a container of about 1 pound of uranium into neighboring Hungary.

Washington has stepped up efforts to assist Ukraine in improving its border controls to prevent the smuggling of illegal weapons and materials and technology for weapons of mass destruction.