Two peacekeepers were killed on Monday after an explosion from an unknown origin destroyed their vehicle near Bani Hayyan in south Lebanon, the UN peacekeeping force UNIFIL said in a statement.
Two other soldiers were wounded in the blast.
Earlier, another Indonesian soldier was killed overnight Sunday into Monday when a projectile exploded by one of the group's positions by the southern Lebanese village of Adchit al-Qusayr.
Another peacekeeper was critically injured at the time.
The death on Sunday was the first among the UN's peacekeeping force in the new war between Israel and Lebanese armed group Hezbollah which erupted on March 2.
"These are two separate incidents and we are investigating them as two separate incidents," UNIFIL's spokeswoman Kandice Ardiel said.
In response to the first death, Indonesia's foreign ministry said on Monday the deceased peacekeeper was one of its citizens and that three others were injured by "indirect artillery fire".
Indonesia condemned the incident and said any harm to peacekeepers is unacceptable while reiterating its condemnation of "Israel's attacks in southern Lebanon".
The Israeli military did not immediately comment on the incidents.
UN Secretary-General Antonio Guterres said attacks on peacekeepers are grave violations of international humanitarian law and may amount to war crimes.
"We strongly condemn these unacceptable incidents - peacekeepers must never be a target," UNIFIL chief Jean-Pierre Lacroix told reporters in a briefing on Monday.
UNIFIL is stationed in southern Lebanon to monitor hostilities along the demarcation line with Israel - an area that is at the heart of clashes between Israeli troops and Iran-backed Hezbollah fighters.
Lebanon was pulled into the war in the Middle East when Hezbollah fired rockets at Israel in solidarity with Iran, two days after Israel and the United States attacked Iran.
Hezbollah's attack prompted a new Israeli ground and air offensive.
More than 1240 people have been killed in Israeli strikes on Lebanon, according to Lebanese authorities.
They include more than 120 children, nearly 80 women and dozens of paramedics.
More than 400 Hezbollah fighters have been killed since March 2, according to two sources familiar with Hezbollah's count.
The Israeli military issued warnings to leave to residents of six villages in Lebanon's western Bekaa region on Monday, in the first such warning for those areas.
The military said the warning was prompted by what it described as militant activity in the area, without providing further details.
Fresh air strikes hit several towns in southern Lebanon on Monday and at least one strike hit Beirut's southern suburbs.
The Israeli military said strikes in Beirut targeted commanders responsible for co-ordination between Hezbollah and Palestinian militant groups.
