The seizure of the Grace 1 by British marines and local police off Gibraltar sparked a diplomatic row, with Tehran summoning the British ambassador. Britain said it impounded the tanker on suspicion of violating EU sanctions against Syria.
US media earlier quoted sources as saying said the British tanker was threatened by armed Iranian Revolutionary Guard boats and the incident was filmed by a US aircraft overhead.
Bill Urban, a US Central Command spokesperson, said the US was aware of the reports, adding: "Threats to international freedom of navigation require an international solution."
The Iranian Revolutionary Guards denied any involvement, saying in a statement that there was "no incident involving foreign vessels in the Strait of Hormuz" in the last 24 hours.
General Joseph Dunford, chairman of the US military's Joint Chiefs of Staff, said Tuesday the US was attempting to build a military coalition to protect shipping in the region.
Washington has accused Tehran of being behind a series of recent attacks against commercial vessels near the Strait of Hormuz.
Last month, President Donald Trump claimed he came close to ordering an airstrike in retaliation against Iran for shooting down a US spy drone near the Strait.
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