Tension between the United States and Iran is intensifying despite signs, however weak it may be, of negotiations.

On Tuesday, the Iranian government said it had written to the US seeking an apology for alleging that Iran backed a plot to assassinate Saudi Arabia's ambassador to the US on American soil. Teheran's riposte was in response to a letter from Washington offering talks.

Last month, the US said that Manssor Arbabsayara, a 56-year-old US citizen holding both Iranian and US passports, and Gholam Shakuri, a member of Iran's Islamic Revolutionary Guard Corps, were charged with sponsoring and promoting terrorism, including plotting to assassinate the Saudi ambassador. Arbabsayara was arrested in the US, while Shakuri remains presumably in Iran.