According to the UN, Israel dropped more than a million cluster bomblets in southern Lebanon during the 34-day war and 40 per cent failed to explode.
It said last summer that the munitions litter homes, gardens and roads across the region.
Unexploded ordnance has killed at least 27 people and wounded more than 140 since the end of the war, including two Belgian soldiers injured on Monday.
The US Arms Export Control Act bans the use of cluster munitions in civilian areas and under the terms of Israel's purchase of cluster bombs from the US, it must agree in writing to restrictions on their use.
Under the Act, if the US government believes that a foreign country has violated agreements over how US-made weapons are used, the administration must draw up a report and send its findings to congress.
Israel says Lebanese civilians were not targeted and were warned in advance by leaflets dropped from aircraft.
'Responsive and transparent'
McCormack would not say how Israel had violated US rules or what action might be taken against Israel if a violation is confirmed.
He said the Israeli government had been "responsive and transparent" in providing information for the report.
He said: "This is not a final judgement ... but we do take our obligations under the law very seriously. We don't flinch from the facts."
Cluster bombs burst into bomblets and spread out over a wide area near the ground.
However, many of the apple-sized bomblets fail to explode on impact, but can do so at the slightest touch at a later date.