(CNN) -- German Chancellor Angela Merkel phoned Pope Benedict XVI Sunday over a Holocaust denier whom the pope welcomed back into the Roman Catholic Church last month.

Neither side seems to have shifted its position over Bishop Richard Williamson, who, shortly before the pope lifted his excommunication, denied the Nazis had systematically murdered six million Jews during World War II.

"It was a very constructive conversation," the German government and Vatican said in a joint statement about the call. Merkel and the pope expressed respect for each others' opinion, the release said -- diplomatic-speak for saying neither side budged.

Merkel demanded Tuesday that the pope firmly reject Holocaust denial: "The pope and the Vatican must make absolutely clear that there can be no denial of the Holocaust," Merkel said.

The Vatican has pointed to several statements by Pope Benedict in the past few years condemning the destruction of European Jewry, including his visits to concentration camps. He has also said he did not know of Williamson's views on the Holocaust when he lifted the excommunication.

"I believe that the historical evidence is strongly against -- is hugely against -- 6 million Jews having been deliberately gassed in gas chambers as a deliberate policy of Adolf Hitler," Williamson said recently in an interview with a Swedish television station, which also appeared on various Web sites after its broadcast. "I believe there were no gas chambers."

Germany's Catholic bishops Saturday called for the expulsion of Williamson, a member of an ultra-conservative group expelled from the Church by Pope John Paul II in 1988.

http://edition.cnn.com/2009/WORLD/europe/02/08/germany.bishop/index.html