An explosive report is due to be published on Monday alleging that Israeli intelligence eavesdropped on telephone conversations made by U.S. Secretary of State John Kerry during Israeli-Palestinian peace negotiations.

U.S. Secretary of State John Kerry climbs up the stairs of the plane to leave Tel Aviv, Israel Tuesday April 1, 2014, continuing on to NATO meetings in Brussels after meeting in Israel with Israeli Prime Minister Benjamin Netanyahu about the Middle East peace process talks. (AP Photo/Jacquelyn Martin, Pool)

US Secretary of State John Kerry leaving Tel Aviv in April

The highly respected German weekly Der Spiegel, in an expose today, quotes “reliable sources” in alleging that at least one other agency listened in.

The report says that spies were able to intercept the calls between Kerry (pictured) and other high-ranking negotiators on both sides because they were made on landlines rather than encrypted equipment.

The calls came during the height of the peace talks last year, which broke down in April, with both sides blaming each other.

Kerry subsequently reported back to the US Foreign Affairs Committee that Israel’s settlement-building was to blame.

If true, the revelations of the eavesdropping “could further damage already tense relations between the US government and Israel,” wrote the Der Spiegel commentary.

The U.S. State Department and Israeli officials declined to comment.