US soldiers from the 2-325 82nd Airborne Division move their equipment into a combat outpost set up in the restive Adhamiyah neighborhood of northern Baghdad. [AFP]

New coils of barbed-wire and blast barriers marked checkpoints that caused traffic bottlenecks. U.S. Apache helicopters whipped the air over parts of the capital where they hadn't been seen before.

But gunfire still rang out across the city, and some residents said they doubted life would get better. "Nothing will work, it's too late," said Hashem al-Moussawi, a resident of the Sadr City Shiite enclave who was badly wounded in a bombing in December.

Underlining the dangers, a U.S. Marine transport helicopter crashed northwest of Baghdad, killing all seven on board ¡ª the fifth American aircraft lost in Iraq in just over two weeks. A military statement did not give a cause for the crash, but a senior U.S. defense official in Washington said the CH-46 Sea Knight did not appear to have been hit by hostile fire. An Iraqi air force officer, however, said the chopper was downed by an anti-aircraft missile. An al-Qaida-linked Sunni group claimed in a Web statement that it was responsible.

At checkpoints that seemed to have been thrown up overnight ¡ª some of them blocking half the lanes of traffic on wide streets ¡ª Iraqi police and army soldiers searched cars at random. Drivers and passengers had to get out and show identity papers.

Adding to the tension, Iraqi army and police convoys fired rounds into the air above motorists, warning them to make way for passing forces. The security troops drove over traffic medians and into incoming traffic.

In a sign of just how dangerous the security mission will be, a three-vehicle Western security company convoy came under fire near Haifa Street, a Sunni insurgent stronghold just north of the Green Zone.

The security men in the armored cars returned fire and quickly exploded green and white smoke bombs for concealment. Minutes later, after the smoke cleared, they sped away. An Associated Press reporter could see bodyguards in the convoy leaning out the car windows and pelting surrounding vehicles with water bottles to persuade drivers to clear the way.

At about the same time, four guards at a nearby building housing state television were shot and killed on the rooftop. An official at Iraqiya television said the men were hit by fire from security company personnel escorting foreign visitors to the Justice Ministry across the street. The television official spoke on condition of anonymity because he was not authorized to speak to the media.

In parts of the city only a few shops were open, a reminder of the fear people have of more car and suicide bombings, which have hit Baghdad with regularity in recent weeks.

Lines outside gas stations stretched for more than a mile, as the city is experiencing its worst fuel crisis in months. Supplies are very low because refining capacity is down, a problem compounded by hoarding.

Gunfire rang out across the capital, and the wail of police and ambulance sirens seemed incessant. The buzz of low-flying U.S. helicopters and growl of fighter jets was nonstop above a new crop of government posters and billboards speaking of Baghdad's struggle.

"Our streets are deserted and our blood is fair game," declared one that showed an empty street strewn with debris from a bombing.

Another billboard showed a young man weeping because he had not reported suspicious activity to authorities. "I should have done the right thing," he says.

Still another billboard message implored: "Be a hero and report suspicious behavior."

Most taxi drivers were refusing to take passengers to areas dominated by the other Muslim sect. Minibus drivers were demanding passengers prove they live in the region to which they wanted to travel.

The streets became nearly deserted well before nightfall, a surprising sign of fear among a population that has lived through wars for much of the past 25 years. Those with money to spare, residents say, are stocking up on fuel for generators and on basic foodstuffs like flour, grains and potatoes.

With electricity available only about two hours a day in much of the city, residents also were buying candles and lanterns. The rickety stands of some outdoor food markets, a favorite target for suicide bombers, stood empty.

"We live hand-to-mouth and don't have money to stock up on anything," said Ibrahim Mohammed, a 78-year-old retired engineer from the Azamiya district. The predominantly Sunni area in north Baghdad was likely to be high on the list of targets in the Baghdad security plan.

"No one goes to work anymore," Mohammed said.

Maj. Gen. William B. Caldwell, the top U.S. military spokesman in Iraq, said Wednesday that the much-awaited Baghdad security operation was finally under way. "The implementation of the (Iraqi) prime minister's plan has already begun and will be fully implemented at a later date, having all the parts and pieces that he wants," he told a news briefing.

The operation is the third attempt by Prime Minister Nouri al-Maliki and his U.S. backers to pacify Baghdad since the Shiite leader came to office in May. The operation, which will involve about 90,000 Iraqi and American troops, is seen by many as a last chance to curb Iraq's sectarian war.

On Tuesday, however, Defense Secretary Robert Gates told U.S. lawmakers the buildup in troops is "not the last chance" to succeed in Iraq and "I would be irresponsible if I weren't thinking about what the alternatives might be."

Many Baghdad residents said they weren't hopeful.

"If this security plan is the same as those we had before, with checkpoints delaying the traffic for hours, then I can tell you now that it will be a failure," said Murtadha Mahdi, a 35-year-old unemployed father of two who lives in Hurriyah, now a predominantly Shiite district in northern of Baghdad that saw some of the worst sectarian fighting late last year.

The security sweep in Baghdad comes nearly a year after the city became the main battlefield of sectarian violence following al-Qaida's bombing of a major Shiite shrine north of Baghdad. Thousands have since died in the capital, victims of Shiite militiamen or suicide bombings blamed on Sunni militants. Thousands more have been displaced from their homes.

"Sunni and Shiite politicians pretend to work for reconciliation, but they curse each other when the news cameras are gone," said al-Moussawi, the Sadr City resident who was hospitalized for two months with severe wounds to his chest, right arm and leg.

The main entrance to the district now has four checkpoints manned by Iraqi police and army troops backed by armored personnel carriers mounted with large caliber guns. Hundreds have been killed in Sadr City in suicide bombings and mortar attacks in recent months.

A giant billboard near the site of a series of attacks ¡ª including the one that wounded al-Moussawi ¡ª displays the pictures of more than 40 victims, many of them children. A woman clad in black is shown weeping, her face buried in her hands.