April 7 (Reuters) - Preliminary findings in a U.N. probe into the deaths of three Indonesianpeacekeepers in Lebanon last month show one was killed by an Israeli tank projectile and two others by an improvised explosive device most likely placed by Hezbollah, the U.N. said on Tuesday.

"These are preliminary findings, based on initial physical evidence," U.N. spokesperson Stephane Dujarric told a news briefing, adding that a full investigation process was continuing that included engagement with the parties concerned.

Dujarric called the incidents "unacceptable" and said they could amount to war crimes under international law. He said the United Nations had requested that the cases be investigated and prosecuted by national authorities to bring the perpetrators to justice.

The Indonesian peacekeepers were killed in two separate incidents in southern Lebanon on March 29 and 30 after a bloody weekend in which Lebanese journalists and medics were also killed in Israeli strikes.

Bombardment in southern Lebanon on Tuesday forced a convoy of humanitarian aid organized by the Vatican's embassy for a besieged Christian town to turn back, a priest in the town told Reuters.

Also on Tuesday, a spokesperson for the peacekeeping force, the United Nations Interim Force in Lebanon, said the Israeli military had blocked a UNIFIL logistics convoy and briefly detained one of its peacekeepers. The spokesperson, Kandice Ardiel, said that any detention of a United Nations peacekeeper was a blatant violation of international law and that the Israeli military had informed UNIFIL it had launched an investigation into the issue.

(Reporting by David Brunnstrom in Washington; Additional reporting by Menna Alaa El Din; Editing by Matthew Lewis)