JERUSALEM (JTA) — Israeli intelligence eavesdropped on telephone conversations made by U.S. Secretary of State John Kerry during Israeli-Palestinian peace negotiations, Der Spiegel reported.

In addition to Israel, at least one other intelligence agency also listened in on the phone calls, the German weekly reported in an article to appear in its Monday print edition. A summary was posted to its English-language website on Sunday.

The article cites unnamed “reliable sources.”

The intelligence agencies reportedly were able to intercept and listen to the calls because some of the calls made by Kerry during the height of the peace talks last year to high ranking negotiators on both sides were done one regular landlines, instead of on encrypted equipment.

“Revelations of the eavesdropping could further damage already tense relations between the US government and Israel,” der Spiegel observes.

Both the U.S. State Department and Israeli officials declined to comment to Der Spiegel.