Tanzania is seriously investigating allegations that it re-flagged Iranian oil tankers in defiance of sanctions.
A US congressman alleged last week that up to 10 Iranian oil tankers had been re-registered in Tanzania.
But the East African nation's foreign minister said so far there was no evidence to suggest this was true and he appealed for help with the probe.
The US and the European Union have just tightened sanctions on Iran over concerns about its nuclear programme.
On Sunday, a complete European Union oil embargo on Iran came into effect - in response to US legislation, which sanctions any entity that deals with Iran's Central Bank.
Tanzania's neighbour Kenya on Wednesday said it had cancelled plans to import crude oil from Iran following threats of sanctions.Flags of convenience
Howard Berman, who chairs the US House of Representative's Committee on Foreign Affairs, wrote to President Jakaya Kikwete last Fridayabout his concerns that Tanzania had allowed National Iranian Tanker Company vessels to re-flag.
In a briefing to journalists on Thursday, Tanzania's Foreign Minister Bernard Membe said that if any Iranian ships had been re-flagged, they would be removed from Tanzania's shipping register."If Tanzania were to allow Iranian vessels to remain under Tanzanian registry, we in the Congress would have no choice but to consider whether to continue the range of bilateral US programs with Tanzania," the US politician warned.
But he had received assurances from the Iranian embassy and a shipping agent in Dubai that no Iranian ships have been re-flagged.
"We're calling on the international community, particularly the United States and European Union, to determine whether or not the statements... are correct or not," Mr Membe said.
"If it is confirmed that the ships flying Tanzania's flag are indeed from Iran, we will take steps to deliberately obliterate the registration."
The BBC's international development correspondent Mark Doyle says the use of so-called flags of convenience is common, though controversial, for legitimate trade.
But the flags are also sometimes flown as a way of trying to hide the origin of vessels and their cargo.
On Wednesday, Sierra Leone's government said it would ban any ships flying its flag from violating international sanctions.
The move came after reports that two ships, using a Sierra Leonean flag of convenience, had carried Syrian oil products against the aims of US and European sanctions imposed to try to prevent President Bashar al-Assad's access to foreign exchange.