By Kacper Pempel

SOBIBOR, Poland, Sept 18 (Reuters) - Archaeologists working at the site of the Nazi concentration camp at Sobibor, in eastern Poland, say they have uncovered previously-hidden gas chambers in which an estimated quarter of a million Jews were killed.

German forces tried to erase all traces of the camp when they closed it down following an uprising there on Oct. 14, 1943. The Nazis demolished the gas chambers and an asphalt road was later built over the top.

Archaeologists excavated beneath the road and found lines of bricks, laid four deep, where they believe the walls of the gas chambers used to stand.

They have been able to establish how big the chambers were, information they said would help build up a more precise picture of how many people were murdered at the camp.

"Finally, we have reached our goal -- the discovery of the gas chambers. We were amazed at the size of the building and the well-preserved condition of the chamber walls," said Yoram Haimi, one of the archaeologists.

Haimi said two of his own uncles, who had been living in Paris during the war and were rounded up by the Germans, were among those who were killed at Sobibor.

The archaeologists said among the personal items they had come across buried in the ground near the gas chambers was a wedding ring which carried the inscription, in Hebrew: "Behold, you are consecrated unto me."

EVIDENCE DESTROYED

Historians say that because the Germans razed the camp, and because so few of those detained there came out alive to give testimony, there is less information about how Sobibor operated and the scale of the killing than there is for some other concentration camps.

Polish archaeologist Wojciech Mazurek, who has also been involved in uncovering the site, said the excavations revealed there were eight gas chambers.

"The extermination of people took place there; murder by smoke from an engine that killed everyone within 15 minutes in these gas chambers, in torment, shouting," he told Reuters Television.

"It is said that ... the Nazis even bred geese in order to drown out these shouts so that prisoners could not have heard these shouts, these torments."

According to Israel's Yad Vashem International Institute for Holocaust Research, the 1943 uprising was organized by Jewish civilians at the camp and Jewish officers in the Soviet army who had been taken prisoner and sent to Sobibor.

About 300 people escaped, but most were caught and killed. Those who did not take part in the break out were also killed. At the end of World War Two, about 50 escapees were left alive.

The research project at Sobibor is being carried out in coordination with the Israeli-based Yad Vashem International Institute for Holocaust Research, the German-Polish Foundation, and the Majdanek State Museum, near the Polish city of Lublin. (Additional reporting by Robert Furmanczuk and Michal Ratynski; Writing by Christian Lowe; Editing by Crispian Balmer)