BAGHDAD — U.S. military deaths plunged in May to the lowest monthly level in more than four years and civilian casualties were down sharply, too, as Iraqi forces assumed the lead in offensives in three cities and a truce with Shiite extremists took hold.

But many Iraqis as well as U.S. officials and private security analysts are uncertain whether the current lull signals a long-term trend or is simply a breathing spell like so many others before.

At least 21 American troops were killed in May — four in non-hostile incidents. That's one more than the lowest monthly figure of the war set in February 2004.

U.S. commanders also warn the relative peace is fragile because no lasting political agreements have been reached among the Shiite, Sunni and Kurdish communities.

Talks on returning Sunnis to the government broke down this week, and tensions among rival Shiite parties remain high despite a May 11 truce that ended weeks of bloody fighting in Baghdad's Sadr City district.

Iraqis have experienced lulls in the past — notably after the January 2005 elections — only to see violence flare again.

"The security situation is much better than in the past three or four months, and I am making more money now," said Falih Radhi, who runs a food store in eastern Baghdad. "Despite this, I have a feeling that this positive situation won't last long and that violence may come back again."

Nevertheless, the figures for May are encouraging, especially coming as the United States continues withdrawing the nearly 30,000 reinforcements that President Bush sent to Iraq early last year to curb the wave of Shiite-Sunni slaughter.

All five of the "surge brigades" rushed to Iraq last year will be gone by July, lowering the troop strength to about 140,000, U.S. officials say. There are currently about 155,000 U.S. troops in Iraq.

Meanwhile, Iraqi deaths were down, too.

At least 532 Iraqi civilians and security troopers were killed during the month, according to figures compiled by The Associated Press from Iraqi police and military reports. That's down sharply from April's figure of 1,080 and the lowest monthly total this year, according to the AP count.

IRAQI FORCES LEAD

Last Sunday, military spokesman Rear Adm. Patrick Driscoll said the number of attacks in the previous week fell to a level "not seen since March 2004," although he did not give specific figures.

At the same time, Iraqi forces have taken the lead in offensives against the Sunni extremist al-Qaida in Iraq in the northern city of Mosul and against Shiite militiamen in Baghdad and Basra in the south.

U.S. and coalition forces assumed a support role in the three offensives, enabling them to avoid higher casualties which would have been expected had they been doing all the fighting.

With the trends looking positive, the top American commander in Iraq, Gen. David Petraeus, said in Washington last week that he is likely to recommend further troop cuts in Iraq but won't promise more details until fall — as the U.S. presidential election campaign is approaching its climax.

But U.S. officials and private security analysts warn against rapid withdrawals and optimistic forecasts.

Former Pentagon analyst Anthony Cordesman wrote this week that despite some improvements among Iraqi forces, both Iraqi and U.S. officials continue "to sharply exaggerate the real-world readiness" of the country's army and police.

Petraeus himself said it's unlikely that Iraqi security forces can take the lead in all 18 provinces this year, as was recently predicted by the Pentagon.

"The overall trend in Iraq is positive, but we should be skeptical about overly optimistic assessments that we've 'turned the corner' in Iraq," said Eric Rosenbach of the Belfer Center for Science and International Affairs and a former staffer of the Senate Select Committee on Intelligence.

"It's more appropriate to say that we have a long road ahead of us rather than we've turned the corner."

SUNNIS STILL ACTIVE

About half the U.S. deaths in May occurred in Sunni areas, showing that Sunni insurgents remain active, even though thousands of Sunnis have agreed to work with the Shiite-dominated government.

Top leaders of Shiite militant groups that fought the Americans and Iraqis for weeks in Sadr City have escaped, the U.S. military says, presumably to regroup and fight again.

"If you look at it in terms of a video recorder, a lot of the groups have pushed pause, but that's not to say they can't push play again," said Nathan Freier, an analyst at the Center for Strategic and International Studies in Washington.

That possibility is greatest within the major Shiite community, where anti-American cleric Muqtada al-Sadr is competing for power against parties that have worked with the United States while maintaining ties to Iran.

Al-Sadr declared a cease-fire last August which largely held until late March, when two rival Shiite parties encouraged the government to move against Shiite militias in Basra.

That triggered an uprising that spread across the Shiite south to Baghdad, where militiamen rocketed the U.S.-protected Green Zone daily.

Fighting ended after Shiite mediation, some facilitated by Iran. The deals enabled Iraqi security forces to extend control in former militia strongholds of both Baghdad and Basra.

But the March fighting broke out because al-Sadr believed his Shiite rivals were trying to weaken his movement before provincial elections this fall.

Those elections are now expected to slip one month to November, and already many Sadrists are complaining that their rivals are again using the truce to arrest and intimidate their followers.

"If some of these constituencies don't get what they want at a rate they find acceptable, they will increase resistance," Freier said. "This is a window of opportunity that will be squandered or capitalized on. And it's in the hands of the Iraqis to capitalize on it."

AP writers Carley Petesch in New York and Hamid Ahmed in Baghdad contributed to this report.