Powerful Shiite cleric Moqtada al-Sadr has vowed to "shake the ground" under the feet of advancing Sunni militants, as Iraqi premier Nouri al-Maliki warned rivals against exploiting the crisis to sideline him.
Sadr, whose movement long battled US forces during Washington's nearly nine-year war in Iraq, also voiced opposition to American military advisers meeting Iraqi commanders.
"We will shake the ground under the feet of ignorance and extremism," Sadr said from the Shiite holy city of Najaf.
Iraq's flagging security forces were swept aside by the initial jihadist push, but have since at least somewhat recovered. And while Sadr's Mahdi Army militia remains officially inactive, fighters loyal to the cleric have vowed to combat the militant advance.
The cleric demanded "new faces" in a unity government following April 30 elections that saw incumbent Maliki emerge with by far the most seats, albeit short of a majority.
Washington has pressed for Iraq's fractious political leaders to unite in a national emergency government. On Wednesday Sadr brushed off Maliki's insistence that such a move would be a "coup against the constitution and the political process".
Sunni tribal leaders have also called for a government to be formed that ignores the April poll, which they describe as a sham. But Iraqi leaders have shown little sign of coming together.
US officials said they believed Maliki was still committed to opening a process on piecing together a government on July 1.
"I think there's just been a little confusion about what he was ruling out here," State Department deputy spokeswoman Marie Harf said.
Washington has stopped short of calling for Maliki to go, but feels he has squandered the chance to rebuild Iraq since US troops withdrew in 2011.