The United Nations said unexploded cluster bombs — anti-personnel weapons that spray bomblets over a wide area — litter homes, gardens and highways in south Lebanon.
"We are definitely looking into these allegations and we'll see where they lead," State Department spokesman Gonzalo Gallegos said Friday.
The inquiry will determine whether the munitions were used and if so, how, Gallegos said.
A spokeswoman for the U.N. Mine Action Coordination Center refused to comment on the investigation. She said that it's not illegal to use the cluster bombs against soldiers or enemy fighters, but the Geneva Conventions bar their use in civilian areas.
Relief organizations and the U.N. mine office have reported finding evidence that Israel used three types of U.S.-made cluster bombs during the 34-day war with Hezbollah militants, during which both sides fired rockets into populated areas.
The U.N. mine office said in a report Wednesday that it had found hundreds of bomblets of the types made by the United States among unexploded ordnance recovered in nearly 250 locations in southern Lebanon.
Israel also makes its own cluster munitions.
The State Department investigation will look at whether Israel's use of U.S.-made cluster bombs violated long-standing secret agreements about how U.S.-supplied weaponry will be used.
The investigation could lead to sanctions if Israel violated those agreements.
Gallegos had no estimate of how long the inquiry by the department's Office of Defense Trade Controls would take.
The Israeli army said all weapons it uses "are legal under international law and their use conforms with international standards."
Cluster bombs are typically used against tanks and explode upon impact with steel. In the conflict in Lebanon, the shells were fired into urban and rural areas where Israel thought Hezbollah guerrillas might be hiding. Many hit the ground or pavement and did not explode.
Israel said it was forced to hit civilian targets in Lebanon because Hezbollah fighters were using villages as a base for rocket-launchers aimed at Israel. At least 850 Lebanese and 157 Israelis died in the fighting.
The Bush administration repeatedly warned Israel to avoid civilian casualties during the cross-border war, but refrained from direct criticism of Israeli tactics.
A spokesman for the Israeli Embassy in Washington said the State Department had not notified the embassy of any pending investigation.
"We have not been informed, and once we are informed we'll look into it," spokesman David Siegel said.
A congressional investigation found Israel improperly used U.S.-made cluster bombs during its 1982 invasion of Lebanon. The Reagan administration then imposed a six-year ban on further sales of the weapons to Israel.