Israeli agents were eavesdropping on US Secretary of State John Kerry as he was mediating last year’s futile Middle East peace talks, German magazine Der Spiegel has revealed.

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Kerry made reviving diplomatic ties between Israel and the Palestinian Authority after a three-year freeze a central priority in the early months of his term as secretary of state. In June 2013, direct talks started, only to collapse completely months later, as the differences overwhelmed whatever desire for an agreement the parties had brought to the negotiations table.

But when the delegations were just starting their doomed effort in the US, Kerry was busy mediating. Some of the diplomatic calls with his counterparts in the Middle East were made not on secure, scrambled channels but on regular phone lines. The calls were intercepted by the Israeli secret services, Der Spiegel said Sunday, citing intelligence sources.

In addition to the Israelis, who used the eavesdropped information to get leverage during the talks, at least one other secret service was monitoring Kerry’s calls, Der Spiegel added.

The German magazine said the revelations could hurt already tense US-Israeli relations amid the ongoing offensive in the Gaza Strip.

Last week, Kerry made another attempt to mediate the current crisis, but failed to secure a lasting ceasefire.