DUBAI (Reuters) - Iran welcomed on Sunday initial results that showed the approval of an Islamist-backed constitution in Egypt, the Iranian Students' News Agency (ISNA) reported.

The Islamist Muslim Brotherhood, which propelled Egyptian President Mohamed Mursi to power in a June election, said an unofficial tally showed 64 percent of voters backed the charter after two rounds of voting that ended with a final ballot on Saturday.

Iran's Foreign Ministry spokesman Ramin Mehmanparast congratulated the Egyptians on the approval of the constitution, adding that Tehran regarded it as "a decisive step towards democracy" in Egypt, according to ISNA.

Diplomatic relations between Tehran and Cairo broke down after Iran's 1979 Islamic revolution over Egypt's support for the overthrown shah and its peace agreement with Iran's arch-enemy Israel.

Since the fall of President Hosni Mubarak, there have been signs of warming relations. In August, Mursi made the first visit to Tehran by an Egyptian leader in more than 30 years. The two countries have not officially upgraded ties.

"People's participation (in the referendum) will be a great support for the Egyptian government so that in the future, it can take more steps in achieving the great Islamic and revolutionary goals of the Egyptian people," Mehmanparast added.

(Reporting by Zahra Hosseinian; Editing by Stephen Powell)