Hamas said Sunday that two members were killed in a mysterious late-night blast, and the Palestinian militant group said it was still trying to determine who was behind the attack.
Lebanon's state-run news agency said Saturday's explosion was caused by three bombs tied to each other and placed under a car believed to belong to a Hamas official. The blast took place near Hamas offices south of Beirut.
Hamas, which governs the Gaza Strip but has exiled leaders in Lebanon and Syria, identified the men as Basil Juma, 29, and Hassan Hadad, 21.
The statement did not specify the men's role in the group, but it referred to them as "fighters" _ usually a reference to low-level members of the group's armed wing.
Osama Hamdan, a senior Hamas representative in Lebanon, said the bomb targeted "offices used by the group with a living quarters for bodyguards."
Lebanon houses about a dozen Palestinian refugee camps with some 200,000 registered refugees.
Explosions in the area, a stronghold of Lebanon's Shiite Hezbollah militant group, are rare. Hezbollah has its own arsenal with tens of thousands of rockets and missiles, which it says it needs to fight off any threat from Israel.
A country notorious for its years of kidnappings, car bombs and political assassinations, Lebanon has seen greater stability recently and earlier this year formed a unity government that includes Hezbollah. But regional tensions and strains within this tiny nation remain.
Also Saturday, U.N. peacekeepers found "a significant quantity of explosives" in southern Lebanon, just 2.5 miles (4 kilometers) from the Israeli border.
The UNIFIL peacekeeping force announced the find Sunday, but offered no further details. The explosives were found in Khiam, where a blast in 2007 killed six U.N. peacekeepers.
In Israel, government spokesman Mark Regev accused Hezbollah of being behind the arms cache, and said the guerrilla group, with backing from Iran, was violating a U.N.-brokered cease-fire that ended a monthlong war in 2006.
"Israel has expressed our concern over a period of months and even longer as to the Hezbollah rearmament. There is no doubt that the leadership in Tehran, while brutalizing its own population, is continuing to systematically arm its proxy in Lebanon."
The unrest comes as Hamas and Israel are engaged in sensitive negotiations for a prisoner swap in a bid to free a long-held Israeli soldier. Both sides have been taking steps to keep tensions at a minimum as Egyptian and German mediators try to forge a deal.
AP Writers Diaa Hadid in Gaza City and Ian Deitch in Jerusalem contributed to this report.