Lebanese Foreign Minister Gebran Bassil made the announcement Monday during a joint press conference with his Iranian counterpart, Mohammad Javad Zarif.
The two-day Warsaw conference begins Wednesday and will be co-hosted by the U.S. Some 80 countries have been invited, including Israel. The conference is widely seen as aimed at isolating Iran, which was not invited.
Lebanon has been following a policy of disassociation from regional conflicts. Iran enjoys wide influence in the country through the Hezbollah militant group, which along with allied parties holds a majority in parliament.
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Lebanon is technically at war with Israel, and Lebanese officials avoid conferences where Israelis are present.
The conference will be attended by Prime Minister Benjamin Netanyahu, as well as U.S. President Trump's son-in-law and senior adviser, Jared Kushner.
Last week, the Palestinian Authority also declined the invitation, which came after Palestinian leadership made it public that they were not invited. The PA's decision not to participate in the conference is line with its policy of not engaging with the Trump administration since the American embassy in Israel was moved to Jerusalem in December 2017. In past months, the Trump administration has also cut more than half a billion dollars in aid to Palestinians.