BAGHDAD (AP) – Loyalists of radical Shiite cleric Muqtada al-Sadr called on the Iraqi government Saturday to hold a public referendum on a long-term security deal with the United States.

Widespread opposition to the deal has raised doubts that negotiators can meet a July target to finalize a pact to keep U.S. troops in Iraq after the current U.N. mandate expires at the end of the year.

The U.S. military, meanwhile, said an American Marine died Friday in a non-combat related incident in Iraq, pushing the number of Americans killed this month to 21 as May draws to a close.

While the number is not final, it would be the lowest monthly death toll since February 2004, when 20 troops died, according to an Associated Press tally based on military figures.

The Iraqi monthly toll also was down, with 516 violent deaths reported to the AP by police and other officials, the lowest since 375 were killed in December 2005.

Senior Sadrists, including lawmakers Falah Hassan Shanshal and Maha Adel, met in the cleric’s Sadr City office in Baghdad and issued a statement calling on the Iraqi government to stop negotiations with the U.S. and to hold a public referendum on the issue.

Al-Sadr also has called for a referendum along with weekly protests against the deal.

U.S. President George W. Bush and Prime Minister Nouri al-Maliki signed a statement last December on the future of U.S.-Iraqi relations.

U.S. and Iraqi officials began negotiations in March on a blueprint for the long-term security agreement and a second deal, to establish the legal basis for U.S. troops to remain in the country after a U.N. mandate runs out.

Although U.S. officials insist they are not seeking permanent bases, suspicion runs deep among many Iraqis that the Americans want to keep at least some troops in the country for many years. U.S. officials have declined to comment on the talks until the draft is completed. The Sadrists said they “absolutely reject” the agreement and urged Iraqis to continue peaceful demonstrations against it. Tens of thousands rallied against the deal on Friday.

Most of the protesters appeared to be followers of al-Sadr, the hardline Shiite cleric and militia leader whose Mahdi Army battled American and Iraqi troops in Baghdad’s Sadr City district until a truce this month ended nearly seven weeks of fighting. But opposition to the agreement appears to be growing beyond the Sadrist movement, with concerns raised in recent days from a militant Sunni clerical group, the Association of Muslim Scholars, one of Iraq’s most powerful Shiite politicians Abdul-Aziz al-Hakim and Shiite spiritual leader Grand Ayatollah Ali al-Sistani.

The U.S. military has continued to target what it calls Iranian-backed Shiite militia factions, warning key leaders have fled to other areas as American and Iraqi forces closed in on them in Sadr City.

American troops acting on tips in eastern Baghdad on Saturday captured a suspect believed to be a key assistant to one of the fugitive militia leaders, according to a statement. The man captured was accused of kidnapping and managing funds for the so-called special groups.

“Intelligence shows that as Iraqi security or coalition forces come close to capturing senior Special Groups leaders, the criminals flee and hide until the situation calms,” military spokesman U.S. Army Capt. Charles Calio said.

Tensions also rose when Nassar al-Rubaie, the leader of the Sadrist bloc in parliament, was stopped at a police checkpoint outside Diwaniyah, south of Baghdad.

The six-car convoy, which was en route from opening a new office in Basra to the holy city of Najaf, was held up for nearly two hours without reason before being allowed to proceed, al-Rubaie told AP in a telephone interview. “We call upon the government to stop the harassment faced by the Sadrists every day and we demand that those responsible for this be tried,” al-Rubaie said.

Police Col. Asaad Ali, the director of the Diwaniyah operations center, said police were under orders not to allow gunmen in the city so had stopped al-Rubaie’s armed guards. But he said a patrol was sent to safely escort the convoy on its way out of the province to Najaf.

Separately, French Foreign Minister Bernard Kouchner toured the ancient ruins of the biblical city of Ur, near Nasiriyah, about 320 kilometers (200 miles) southeast of Baghdad, after arriving for his second visit to Iraq in less than a year.

“This visit … is a message of peace and cooperation and a chance to discuss any future French contribution to rebuilding Iraq,” Kouchner told reporters through an Arabic translator.

Kouchner, the co-founder of the Nobel Prize-winning aid group Medecins Sans Frontieres, or Doctors Without Borders, and a former U.N. administrator for Kosovo, also planned to travel to Baghdad and the northern city of Irbil to open a French consulate in the semiautonomous Kurdish region.

In the capital, he was to meet “Iraq’s most senior officials as well as representatives of civil society,” the foreign ministry said. The trip, coming just before France takes over the rotating European Union presidency, “testifies to France’s renewed political involvement with Iraq and the Iraqi people,” it said in a statement.