BAGHDAD — The wave of attacks by al-Qaida-led Sunni extremists that have killed thousands of Iraqis this year, most of them Shiites, is provoking ominous calls from Shiite leaders to take up arms in self-defense.

They generally insist they’ll do it legally, under the banner of the security forces. But Iraq’s young democracy is still struggling, nearly two years after U.S. troops withdrew, and the specter of armed Shiite and Sunni camps revives memories of the sectarian fighting that took the country to the brink of civil war in the mid-2000s.

Since April, bombings and shootings have killed more than 5,500 people. Averaging at least two a week, they target outdoor markets, cafes, bus stations, mosques and pilgrimages in Shiite areas.

Iraq’s Shiite prime minister, Nouri al-Maliki, said he wants help in quelling the violence.

Departing for Washington, he appealed for quicker delivery of offensive weapons such as helicopters that Baghdad says it needs.

He stressed that a “deeper security relationship” with the U.S. is needed.

Since late December, Iraq’s minority Sunnis have been protesting what they perceive as discrimination and tough antiterrorism measures against them by the Shiite-led government. The Sunni attacks followed a government crackdown on a Sunni protest camp in the northern town of Hawija in which 44 civilians and one member of the security forces were killed, according to U.N. estimates.

Now high-profile calls are being made for Shiites to play a role in their own defense by creating armed “popular committees,” attached in some form to the regular security forces. The idea raises the specter of some of Iraq’s darkest years following the 2003 U.S.-led invasion that toppled Saddam Hussein’s Sunni-led regime, paving the way for long-repressed majority Shiites to seize power.

Iranian-backed Shiite death squads roamed Baghdad from 2006-2008, killing Sunnis by the dozens and dumping their bodies on the streets or in the river in retaliation for the devastating bombings and suicide attacks blamed on Sunni insurgents.

It was a cease-fire by militia leader and anti-American cleric Muqtada al-Sadr, along with a Sunni revolt against al-Qaida in Iraq and a series of U.S.-Iraqi offensives that helped quell the bloodshed. While Iraqis continued to face near-daily attacks, they hoped the days of rampant sectarian warfare were behind them. Now a politician, al-Sadr has urged calm among his followers.