Israel probably violated the terms of its arms deals with Washington by using US-made cluster bombs in Lebanon last year, a US government report says.
Thousands of bomblets have been found across Lebanon
The state department looked into Israel's use of cluster bombs in civilian areas of southern Lebanon during its conflict with Hezbollah.
US-made weapons are sold to the Israeli military with restriction on their use.
Cluster bombs can scatter hundreds of small bomblets over a wide area, and their use has been widely criticised.
The International Committee of the Red Cross called for a ban on the use of cluster bombs in populated areas, because of the indiscriminate civilian deaths they caused.
And Amnesty International has criticised Israel for its use of cluster bombs in the final days of the conflict.
The US government has now sent a preliminary report on its investigation into the matter to the US Congress.
State Department spokesman Sean McCormack said the report delivered to Congress was not a "final judgement".
He refused to elaborate on the report's conclusion, but said the terms agreed with Israel for the use of US-supplied munitions were "likely" to have been violated.
"This is a preliminary finding and because it also involves the agreements about use (of munitions), which are classified, I cannot get into the details," he added.
Congress will now consider the report before deciding whether to take any further action against Israel.
Israel has consistently maintained it uses cluster bombs in line with international law, although in November the military said it would investigate how the bombs were used during the conflict.
Israel denies breaking any agreement with the US and says it is co-operating with investigators.
"Israel takes the concerns raised by the US very seriously. In our response, we have been as detailed, as forthcoming and transparent as possible," said government spokesman Mark Regev.
The US is Israel's biggest military donor, offering about $2bn of aid and assistance each year.