The US government has accused Iran of engaging in what it called provocative and potentially hostile behaviour.
US officials claim that armed Iranian boats confronted three US warships in the strategic Strait of Hormuz on Sunday. The US says that they radioed a threat to the vessels.
Iran has played down the encounter, which comes just days before President George W Bush travels to the region to boost the Israeli-Palestinian peace process.
US Defence Secretary Robert Gates said the incident was 'a matter of real concern'.
However, Iranian Foreign Ministry spokesman Mohammed Ali Hosseini called it 'an ordinary occurrence which happens every now and then for both sides.'
