The comment was reportedly made in a meeting late Tuesday between Senator Obama and Democratic members of the House of Representatives, following the presidential contender's return from an overseas tour that included Israel.
"Nobody said this to me directly but I get the feeling from my talks that if the sanctions don't work, Israel is going to strike Iran," an attendee at the meeting quoted Obama as saying, according to ABC.
Neither the Obama campaign nor his Senate office had any immediate comment.
Iran has been slapped with three sets of UN Security Council sanctions over its refusal to halt uranium enrichment, a process that makes nuclear fuel but also the core of an atomic bomb.
Iran's supreme leader Ayatollah Ali Khamenei said on Wednesday that his country would not retreat in the face of demands by world powers for Tehran to stop the enrichment.
Obama also told Tuesday's meeting that Arab states understood that a nuclear Iran would be a "game changer" for the entire region because of likely Israeli action, another attendee cited by ABC said.
Visiting Paris as part of his foreign tour, which also took in Jordan, Obama said Friday that Iran should not wait for the next US president to be elected to resolve its nuclear dispute with the West.
"Iran should accept the proposals that President (Nicolas) Sarkozy and the EU… are presenting now. Don't wait for the next president," he said at a joint press conference with Sarkozy.
