Iran has begun five days of large-scale air defence exercises aimed at testing its ability to protect the country's nuclear facilities from possible attack. The manouvres are the latest in a series intended to show Iran's ability to counter threats to its nuclear programme.The head of Iran's air defence said the aim of the exercises was to thwart aerial reconnaissance and air attacks. Another official warned Tehran would retaliate with a missile strike on Tel Aviv, if it was attacked by Israel.Iran is under intense pressure over its nuclear programme, which critics say is intended to produce nuclear weapons. The US and Israel have not ruled out the prospect of a military attack to prevent Iran developing nuclear bombs. Tehran insists its programme is peaceful.