Head of Iraq's Sadrist movement, cleric Moqtada al-Sadr announced on Sunday his shift to the "national opposition" for a period of no less than 30 days after attempts to form a new government failed.

In a series of tweets, he said he was "honored" to have succeeded in forming the largest bloc in parliament, away from disputes over shares and quotas.

"I was honored to have relied on myself and to not be beholden to foreign sides," he added.

"I was honored that I was not forced to resort to the judiciary to facilitate the needs of the people and the formation of the government," he continued.

However, Sadr said obstacles thrown by internal and external forces thwarted his efforts to form a national majority government.

"We now how have one choice that we must try, shifting to a national opposition for a period of no less than 30 days," he revealed.

"If the parliamentary blocs, including those who we were honored in allying ourselves with, succeed in forming a government and easing the suffering of the people, then we will bless it," he continued.

"We will have our say" if they don't, he warned.