BAGHDAD (REUTERS) - Iraqi Prime Minister Haider al-Abadi announced on Sunday (Jan 14) his candidacy for the May 12 parliamentary elections to choose a prime minister.

Abadi, a Shi'ite Muslim who led the country in the four-year war against Islamic State in Iraq and Syria (ISIS), said he will seek to form a cross-sectarian block called "the victory alliance" to contest the parliamentary elections, with candidates from other communities.

Abadi took over the premiership in 2014 from Nuri al-Maliki, a close ally of Iran widely criticised by Iraqi politicians for the army's collapse as ISIS militants swept through a third of Iraq.

Maliki, who heads the Shi'ite Dawa party, announced on Saturday he will be running in the elections.

Abadi is a Dawa member but he didn't secure Maliki's endorsement for his candidacy.

Maliki said on Saturday Dawa supporters will be free to choose between his alliance, called "state of law", and Abadi's "victory alliance."

Abadi is credited for quickly rebuilding the army and defeating ISIS in its main Iraqi stronghold, Mosul, last July, with strong assistance from a US-led coalition.

Maliki holds the ceremonial title of vice-president. He remains a powerful political figure as head of the Dawa and the largest political block in the current parliament.

The prime minister's office is reserved for Iraq's majority Shi'ite Arab community under a power-sharing system set up after the 2003 US-led invasion that toppled Saddam Hussein, a Sunni Arab.

The largely ceremonial office of president is reserved for a Kurdish member of parliament. The speaker of parliament is drawn from Sunni Arab MPs.

The parliament is yet to approve the May 12 date for the elections.