John Kerry's new diplomatic bid is aimed at uniting Iraq's fractious leaders in repelling jihadist fighters calling themselves the Islamist State of Iraq and Syria (ISIS), whose lightning offensive has displaced hundreds of thousands and put Iraqi Prime Minister Nouri al-Maliki under intense pressure.

Kerry flew out of Washington on Saturday, and will visit Amman, Jordan, then Brussels and Paris. He is expeced to visit Iraq, although the exact timing of any visit is still not yet known.

Around 20,000 Shiites, many in combat gear, marched through Baghdad with assault rifles and rocket launchers on Saturday in rallies that were called for by Iraqi Shiite cleric Muqtada al-Sadr.

The show of force came after al Qaeda breakaway group the Islamic State of Iraq and Syria (ISIS) and allied Sunni rebels captured a Syrian border crossing near the Syrian border in the town of Qaim.

Shiites held similar parades in southern cities of Amarah and Basra.

Washington favored Maliki when he became prime minister in 2006, as he promised to crack down on Shiite militias. But in recent months, the prime minister has made increasingly sectarian moves, triggering calls from US leaders to include Sunnis, Kurds, and Christian in Iraq's political process.

UN Secretary-General Ban Ki-moon on Friday warned against any US military intervention, saying that it could backfire, after President Barack Obama announced on Thursday that he was prepared to launch targeted airstrikes against ISIS, if necessary.

bk/rc (AFP, DPA)