BAGHDAD – Iraqi civilian deaths have fallen in Baghdad in the two months since the Feb. 14 start of the U.S.-led offensive, according to an Associated Press tally.

Outside the capital, however, civilian deaths are up as Sunni and Shiite extremists shift their operations to avoid the crackdown.

And the sweeps have taken a heavy toll on U.S. forces: Deaths among American soldiers climbed 21 percent in Baghdad compared with the previous two months.

Since the crackdown began Feb. 14, U.S. military officials have spoken of encouraging signs that security is improving in the capital but have cautioned against drawing any firm conclusions until at least the summer.

Figures compiled by the AP from Iraqi police reports show that 1,586 civilians were killed in Baghdad between the start of the offensive and Thursday.

That represents a sharp drop from the 2,871 civilians who died violently in the capital during the two months that preceded the security crackdown.

Outside the capital, 1,504 civilians were killed between Feb. 14 and Thursday, April 12 compared with 1,009 deaths during the two previous months, the AP figures show.

”We know this increased security presence and cooperation from the people is having an impact in Baghdad,” U.S. military spokesman Maj. Gen. William C. Caldwell said this week. ”It is a good beginning, but it is not nearly enough. The violence across the rest of Iraq remains at unacceptable levels.”

U.S. officials have cautioned that numbers alone cannot provide a complete picture of the security situation.

The Baghdad crackdown was designed to provide the Iraqi government with what U.S. officials call a ”secure platform” and to buy time for the country’s religious and ethnically based political parties to agree on key reforms.

So far there has been little progress on that front.

Sunni and Shiite militants remain a potent force — regardless of whether they are slaughtering civilians in the capital at the previous rate.

On Thursday, extremists managed to penetrate the most secure part of the capital — the Green Zone — and launch a suicide attack in the building where the Iraqi parliament meets.

Earlier in the day, a suicide truck bomber heavily damaged a major bridge across the Tigris River, collapsing part of the span into the muddy waters.

Such spectacular attacks may not produce a large number of civilian casualties. But they undermine public confidence — which the U.S. military believes is essential for lasting stability.

”It is not going to be possible to see just how well the resulting mix of capabilities will counter the insurgency until the late spring of 2008 at the earliest,” wrote former Pentagon analyst Anthony Cordesman. ”The various insurgents and hostile groups may be weakened or suppressed early on, but will do their best to react.”

It is unclear why deaths outside Baghdad have increased. However, U.S. military officials say both Sunni and Shiite extremists left Baghdad ahead of the crackdown, instead stepping up their operations in a belt of communities around the capital.

The rise in deaths outside Baghdad may also be partly a result of clashes in Anbar province between al-Qaida extremists and Sunni tribes that have broken with the extremist movement.

For example, at least 52 people were killed Feb. 24 when a suicide truck bomber struck worshippers leaving a Sunni mosque in Anbar after the mosque’s preacher spoke out against al-Qaida.

Also, hundreds of Shiites died last month in a spate of bombings and shootings during a religious holiday — including 120 Shiite pilgrims killed by a pair of suicide bombers in Hillah.

One key finding of the figures: Although civilians deaths are down in the capital, a careful analysis of the figures shows that sectarian tensions remain high.

Of the 1,586 civilians killed in Baghdad since the start of the crackdown, more than half — or 832 — appear to have been the victims of sectarian death squads. Their bodies were found scattered around the city. That number represents a significant drop from the 1,754 bodies found in the capital during the two months before the crackdown, according to AP figures. Still, the figure shows that the security crackdown has been unable to stop death squads entirely.

Furthermore, the number of civilians killed by suicide bombers has risen in Baghdad — 352 during the crackdown compared with 279 in the two months before.

Suicide bombings are considered the signature attack of Sunni religious extremists, including al-Qaida in Iraq. And most of the suicide attacks occurred in largely Shiite areas of the capital, indicating attacks on Shiites by Sunnis.

The AP count includes civilians as well as government officials and police and security forces, and is considered a minimum based on AP reporting.

The United Nations had been releasing monthly civilian casualty figures compiled from information received from the Iraqi Health Ministry, hospitals across the country and the Medico-Legal Institute in Baghdad.

However, the U.N. office in Baghdad has not released a casualty report since late January. U.N. officials in Baghdad have been saying for weeks that new figures would be released soon and have offered no explanation for the delay.

Iraqi officials had complained that the U.N. figures were too high.