UPDATE 2-Iran leader orders Muslims to defend Palestinians

(Adds quotes, background)

By Zahra Hosseinian

TEHRAN, Dec 28 (Reuters) - Iran's Supreme Leader issued a religious decree to Muslims around the world on Sunday, ordering them to defend Palestinians in Gaza against Israeli attacks "in any way possible", state television reported.

Ayatollah Ali Khamenei also declared Monday a day of public mourning in Iran after Israel killed more than 280 Palestinians in two days of air strikes on Gaza.

"All Palestinian combatants and all the Islamic world's pious people are obliged to defend the defenceless women, children and people in Gaza in any way possible," Khamenei said.

"Whoever is killed in this legitimate defence, is considered a martyr," he said in a statement.

A religious decree is an official statement by a ranking religious leader that commands Muslims to carry out its message. While there is no religious and legal force behind it, Khamenei is respected by many Iranian and non-Iranian Shi'ites.

Iran refuses to recognise Israel, which accuses Tehran of supplying Hamas Islamists with weapons. Iran denies the claim, saying it only provides moral support to the group.

Israeli leaders said their campaign was a response to almost daily rocket and mortar fire by Gaza militants that intensified after Hamas ended a six-month ceasefire a week ago.