Police hunting smugglers of arms from the former Soviet Union are baffled by the rising trade in "red mercury," a mysterious substance touted as a component of nuclear weapons.

They say the substance sells for up to $175,000 a pound. But many Western scientists have never heard of it and are skeptical of its supposed application in atomic warheads."Since the beginning of the disintegration of the former Soviet Union we have stopped several attempts by smuggling networks to transport through Bulgaria materials applied in nuclear arms production," a senior officer told Reuters.

Most of the seized shipments contained red mercury, a compound of white mercury and antimony that is highly combustible and might have a use in making missiles, he said.

Police showed a Reuters correspondent a beer bottle-sized capsule of the red fluid, embedded in a double metal container bearing Soviet military symbols.

"We confiscated containers of red mercury produced in the former Soviet Union, which are under investigation by our experts," one police official said.

In December, Bulgarian police confiscated a shipment containing red mercury and a conventional warhead from a Soviet SS-19 missile. Several other shipments of red mercury were seized during the last 40 days.

Unlike conventional arms contraband, red mercury is very difficult to detect. It is poured into small containers, which in turn can be hidden in anything, including children's toys.

According to a source who requested anonymity, smugglers offer it in two varieties - a rose-colored powder, which is not radioactive, and a dark red liquid of very high density, which is presumed to be radioactive.

The substance is said to have a wide span of applications, from missile warheads and gravity bombs to purification of uranium.

But Bulgarian scientists said it was still not clear how exactly the substance could be used.

"We know only that the former Soviet Union and another three countries are producers, but its properties are unclear," one physicist told Reuters.

Interior Minister Yordan Sokolov said the embassies of Russia, Britain and the United States in Sofia had been officially asked for more information about the properties of red mercury.

"The three embassies gave unanimous answers - no one knows how it is used," he said.