Steinmeier told reporters in Berlin on Monday that Donald Klein, 53, was released from a Tehran prison and was with German officials.

I am very happy and very relieved that we managed to secure the release of Mr Klein today," he said.

"His long and unfair imprisonment has weighed on our relations with Iran. I am therefore glad that the Iranian government has now agreed to let him go," Steinmeier added.

Klein's release comes less than two weeks after a Frenchman jailed with him, Stephane Lherbier, was freed from Tehran's Evin prison after serving most of his 18-month sentence for straying into Iranian territorial waters during a fishing expedition.

Lherbier was the skipper of a boat taking Klein on a trip from the United Arab Emirates across the Gulf.

Misled by maps

The two men, who were arrested in Nov. 2005, said they had been misled by Emirati maps showing the waters as Emirati, but were both sentenced to 18 months in jail two months later.

Lherbier was freed on Feb. 28. France said he was granted clemency by Iran's supreme leader Ayatollah Ali Khamenei.

After his release, the Frenchman and the German foreign ministry urged Tehran to free Klein as well.

Klein's wife had urged the Iranian authorities to grant her husband clemency, saying he was in fragile health.