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PARIS, July 17 (Reuters) - France, Britain and Germany told Iran on Thursday they wanted Tehran to immediately resume diplomatic efforts over its nuclear program and warned they would reimpose U.N. sanctions if no concrete steps were taken by the end of the summer.

The foreign ministers of the three European countries, along with the EU's foreign policy chief, held their first phone call with Iranian Foreign Minister Abbas Araghchi since Israel and the United States launched airstrikes in mid-June targeting Iran's nuclear program.

A French diplomatic source said following the phone call that the ministers called on Iran to immediately resume diplomatic efforts to reach a "verifiable and sustainable" nuclear agreement.

The three countries, along with China and Russia, are the remaining parties to the 2015 agreement with Iran, which lifted sanctions on the country in exchange for restrictions on its nuclear program.

The UN Security Council resolution enshrining the agreement expires on October 18, and under its terms, previous UN sanctions can be reimposed. This process will take approximately 30 days.

The Europeans have repeatedly warned that unless a new nuclear agreement is reached, they will trigger the "snapback mechanism," which would reimpose all previous UN sanctions on Iran if it is found to have violated the terms of the agreement.

"The ministers also confirmed their intention to use the so-called snapback mechanism if there is no tangible progress toward such an agreement by the end of the summer," the diplomatic source said.

The source did not clarify the nature of the tangible progress referred to.

Since the airstrikes, inspectors from the UN's International Atomic Energy Agency have left Iran. While Iran has indicated its openness to diplomatic efforts, there are no indications that a sixth round of nuclear talks between Washington and Tehran will resume anytime soon.

Diplomats say that even if talks resume, reaching a comprehensive agreement before the end of August, the deadline set by the Europeans, appears unrealistic, especially given the absence of inspectors on the ground to assess what remains of Iran's nuclear program.

Two European diplomats said they hope to coordinate strategy with the United States in the coming days, with the goal of potentially holding talks with Iran soon.

(Prepared by Mahmoud Salama for the Arabic edition)