A senior aide to Israeli Prime Minister Benjamin Netanyahu said that Israel knows more than Congress does about the elements in the emerging U.S-negotiated deal over Iran’s nuclear program.

“We know many details from the agreement being put together, details that we feel members of Congress are unaware of,” the official told reporters aboard Netanyahu’s plane en route to Washington Sunday, according to multiple Israeli media outlets.

“According to the information we have, the deal currently taking shape will leave Iran with the capability to build a nuclear weapon, if [Iranian Supreme Leader Ayatollah Ali] Khamenei make a decision to do so,” said the official, who was not named.

While in Washington, Netanyahu will speak to the American Israel Public Affairs Committee’s annual conference on Monday before his address to a joint session of Congress Tuesday. Before leaving Israel, he described his trip as “fateful” and “historic.”

Ynet reported that the Israeli official said Netanyahu would disclose some of the elements of the emerging agreement with Iran in his address to Congress.