Human Rights Watch called Monday on both the Syrian regime and rebel forces to end "indiscriminate cross-border attacks" on areas in Lebanon, warning they violate international law.
Fire from the conflict in neighbouring Syria has regularly spilled over into Lebanon, killing two people in the most recent incident on April 14. Both rebel forces and regime troops have reportedly been involved in such fire.
"All parties to the conflict in Syria should stop indiscriminate cross-border attacks on inhabited areas in Lebanon," the New York-based rights group said.
It noted that a rebel group claimed responsibility for the April 14 incidents, and that Syrian government forces were believed to be responsible for earlier fire in July 2012 that also killed two Lebanese civilians, wounding others.
The group said it had investigated the April 14 incident, and found no evidence that military assets were targeted, suggesting "these attacks were indiscriminate and therefore violate the laws of war."
Similarly, the group said, areas hit by Syrian regime fire did not appear to contain any military targets.
"Even if fighters are present in Lebanon, there is no excuse for any warring party to conduct indiscriminate strikes on residential areas," the group's deputy Middle East director Nadim Houry said.
"All sides need to take all feasible precautions to protect civilians."
Officially, Lebanon is observing a neutral stance towards the conflict pitting rebels against the regime of President Bashar al-Assad, but the uprising has inflamed tensions in the country's multi-confessional population.
The Sunni-led March 14 opposition movement has backed the uprising, while the powerful Shiite Hezbollah and its allies support Assad.
Damascus has said rebel fighters and weapons are crossing into Syria from Lebanon, while rebel forces accuse Hezbollah of intervening in the conflict by dispatching its fighters to battle alongside government troops.
Lebanon's government has condemned successive incidents in which the conflict has spilled over the border, including alleged Syrian air strikes inside Lebanese territory.
But since the Syrian uprising began in March 2011, there have been numerous deadly clashes along the northern and eastern borders of Lebanon, usually between the Syrian army and armed Syrian or Lebanese groups backing the uprising.
There have also been clashes between armed groups and the Lebanese army seeking to prevent the infiltration of fighters into Lebanon.