"All five entry points on the border between Iran and the Kurdish region have been closed by the Iranian authorities from today," said Jamal Abdallah, spokesman for the Kurdish Regional Government.

There was no immediate confirmation from Iran.

The closure of the border came in protest over the detention of Mahmudi Farhadi, a civilian official, who was arrested from a hotel in Sulaimaniyah on Thursday by the US forces.

On Saturday, Iraqi President Jalal Talabani had demanded his immediate release, saying he was in Iraq as part of a trade delegation.

In a letter to the top US officials in Iraq, Talabani said Tehran had warned of closing the border if Farhadi was not freed.

Talabani said the Iranian was a civilian official who had been visiting with the blessing of both the Kurdish regional government and the authorities in Baghdad.

In a stern statement addressed to General David Petraeus, the head of US forces in Iraq and ambassador Ryan Crocker, he said: "I am informing you of our displeasure over the arrest of the Iranian civilian official without consulting the government of Kurdistan.

"That is a humiliation for the regional administration," said Talabani, who is himself a Kurd.

"You ignored our authority. I ask for his immediate release in order to maintain healthy relations between Iran and Kurdistan and for the prosperity of Kurdistan."

The US military charges that Farhadi was an officer in the covert operations arm of Iran's elite Revolutionary Guards.

But Talabani insisted the detainee was an official on a "commercial mission with the knowledge of the federal government in Baghdad and the government of Kurdistan."

He said the arrest had triggered an angry reaction from Tehran which has "threatened to close its border with the Kurdish region if Mahmudi Farhadi, a civilian employee of Kermanshah (province in western Iran) is not released".

"This will handicap trade in the Kurdish region in this blessed month," he added in a reference to Ramadan, the holiest month in the Islamic calendar.

The US military insists that the detainee is a Quds Force officer, saying they used a photograph to identify him.

Iran's ambassador in Baghdad, Hassan Kazemi Qomi, accused the US military of riding roughshod over Iraq's sovereignty.

"This kind of action violates the sovereignty of Iraq," he said in an interview with AFP.

"This is an example of American mistakes in Iraq."

The US military is also continuing to hold five Iranians it detained in the northern Kurdish city of Arbil in January on suspicion of aiding insurgents.

The five have never been charged and Iran insists they are diplomats.

The US military also briefly detained eight Iranians, including two diplomats, from a Baghdad hotel last month.

But it later released them following protests from the Iraqi government in what it described as a "regrettable incident."