(Reuters) - Iran said Sunday it had received a letter from the U.S. government about the Strait of Hormuz, the crucial oil shipping lane that Tehran has threatened to close if sanctions prevent it exporting oil.

Foreign Ministry spokesman Ramin Mehmanparast was quoted by news agencies as saying Tehran had not yet decided if it would reply to the letter, the contents of which he did not detail.

America's message over the Strait of Hormuz reached us through three channels. It was given to our U.N. representative, the Swiss ambassador conveyed it to the Foreign Ministry and also Iraqi President Jalal Talabani gave the message to Iran, Mehmanparast said, according to the ISNA news agency.

If we deem it is necessary to give a response to America's message, then we will reply to it. The issue is being reviewed by Iran and it will be done in an appropriate way.

Tehran and Washington have had no direct diplomatic relations since 1979 and the Swiss Embassy represents U.S. interests in Iran.

Tehran said on Saturday it had sent a letter to Washington with evidence U.S. intelligence services were involved in the assassination of a nuclear scientist last week.

Washington has said it would not tolerate any closure of the strait - the export route for one third of all seaborne traded oil - with Defense Secretary Leon Panetta has saying such a move would require a response.

Tensions between the two countries have risen in recent weeks. U.S. President Barack Obama signed a bill on New Year's Eve that, if fully implemented, would make it impossible for most countries to pay for Iranian oil.

In addition to threats about Hormuz, Iran has started enriching uranium at an underground bunker and sentencing an Iranian-American citizen to death on spying charges.

Negotiations between the West and Iran over Tehran's nuclear program stalled one year ago.

(Reporting by Hossein Jaseb; Writing by Robin Pomeroy)

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