Despite recent criticisms of his trip to Washington, Israeli Prime Minister Benjamin Netanyahu headed to the U.S. capital on Sunday.
Netanyahu has infuriated the White House and Democrat lawmakers by accepting an invitation by Republicans to speak on Capitol Hill on Tuesday. President Barack Obama had not invited him.
His speech will most likely aim to scuttle an emerging U.S. nuclear deal with Tehran.
Hours before flying to Washington, the Israeli premier made a high-profile visit Saturday to Judaism's holiest prayer site.
Wearing a black skullcap, he rested his hands on Jerusalem's Western Wall while praying briefly, before turning to face a bank of microphones.
Netanyahu expressed his respect for U.S. President Barack Obama, but stood firm on his bid to persuade Congress to fight Obama's Iran plans.
"I believe in the strength of the relations between Israel and the United States," he said. "That strength will prevail over differences of opinion, those in the past and those yet to come."
"As prime minister of Israel it is my duty to care for the security of Israel," he said in remarks broadcast by Channel One TV.
"That is why we strongly oppose the agreement emerging between Iran and the world powers which could endanger our very existence."
Israel believes the U.S. and Tehran are likely to reach an agreement that eases international sanctions on the Islamic republic without applying sufficiently stringent safeguards to stop it developing nuclear weapons.
U.S. Secretary of State John Kerry will leave at the weekend for Switzerland for another meeting with his Iranian counterpart Mohammad Javad Zarif.
The pace and intensity of the talks have increased as global powers gathered under the P5+1 group have sought a deal with Iran to stop it acquiring a nuclear weapon -- something Tehran denies trying to do.
A March 31 deadline for a political framework for the deal is looming, with negotiators saying they will aim to pin down the final technical details by June 30.