US Defense Secretary Robert Gates announced longer tours of duty for US troops in Iraq as violence intensified across Baghdad and living conditions for Iraqis plunged to an all-time low.

In breaking news, it has been reported that a bomb exploded in the heavily fortified Green Zone in Baghdad, where the Iraqi parliament is located, killing at least two Members of Parliament and wounding six others.

Mr Gates told a Pentagon news conference on Wednesday that, effective immediately, all active-duty Army soldiers in Iraq and Afghanistan will serve 15-month tours, extending deployments by three months longer than the standard.

The new policy affects only those serving in the US Army. The Marines will continue to serve their standard seven-month tours and Army National Guard and Army Reserve troops will continue to serve twelve months for each deployment.

A number of units in Iraq have already had their tours extended. The new policy will make deployments more predictable and equitable. “I think it is fair to all soldiers that all share the burden equally,” said Mr Gates. “In the end, I believe this new approach will allow the Army to better support the war effort, while providing a more predictable and dependable deployment schedule for our soldiers and their families.”

Senate Democrats slammed the move, which they said imposed an unreasonable burden on US troops and threatened national security.

“Once again the failures of this administration are being underwritten by our troops; the cost of this will fall on the backs of the brave men and women already serving in harms way, and their families.” said Senator Carl Levin (D-Mich.), chairman of the Senate Armed Services Committee.

Senate Majority Leader Harry Reid (D-Nev.) stated, “Today’s news that active-duty troops' tours in Iraq will be extended to 15 months is another in a long line of examples of how the President's Iraq policies are making us less secure. A bipartisan majority of Congress sees it, the American people see it and military experts see it.”

Senate Republicans expressed similar concerns for national security, as well as the potential erosion of the US military’s ability to recruit new soldiers.

“Having served as Secretary of the Navy when the concept of the all-volunteer force was being developed, and having observed, in the ensuing years, the extraordinary success of that system in providing for America's security, I feel strongly that we must carefully monitor the possible risks to that system that these extensions may generate,” stated Senator John Warner (R-Va.).

Senator Chuck Hagel (R-Neb) said, “Maintaining this tempo of operations will have drastic and lasting consequences for our nations military readiness and unnecessarily endangers our ability to react to any other crisis in the world. We are on a very dangerous course.”

“Our forces are stretched, there’s no question about that,” said Mr Gates. However, he said this is “a difficult but necessary interim step” to provide enough soldiers for President Bush’s decision to send an additional 21,500 troops to Iraq over the next twelve months, while still giving soldiers twelve months between deployments.

The announcement came at an awkward time for Presidential candidate Senator John McCain (R-Ariz.), who delivered an impassioned speech to rally support for the Iraq war and for his campaign, which has been dogged by his strong support for the unpopular war and the recent troop surge.

Meanwhile, the challenges for troops and residents in Iraq show no signs of easing.

On Thursday at dawn, at least one suicide truck bomb exploded on the al-Sarafiya bridge which spans the Tigris River, collapsing two major sections of the bridge and sending several cars hurtling into the river. An Iraqi Interior Ministry Official said that 10 people had been killed in the attack, with a further 26 wounded, most of them rescued from the water by police. The death toll may rise as more cars are searched by divers and pulled from the river.

Although the al-Sarafiya bridge connected a mostly Sunni enclave, Waziriyah, to a mainly Shiite district, Utafiyah, the bridge had nevertheless been regarded as safe to cross because neither neighbourhood was considered volatile. The bridge was built by the British in 1946 and was primarily used by minibuses and commercial vehicles traveling from central Baghdad to markets in the city’s north.

On Tuesday, US and Iraqi troops engaged insurgents in a blazing day-long gun battle in the al-Fadhil district of central Baghdad, a violent Sunni enclave on the Shiite-dominated east bank of the Tigris River. The battle erupted after four US helicopters were fired upon and forced to return to base. At the end of the day, a suspected 14 suspected militants were confirmed dead by the US military, with a further 8 captured and 12 wounded. Many bodies lay in the streets well into the night, with residents and military personnel unable to retrieve them amid the gunfire and chaos.

On Wednesday, the International Committee of the Red Cross (ICRC) said the security situation in Iraq is “disastrous” and the humanitarian crisis is deepening. Hundreds of thousands of terrified residents are known to have fled their homes.

The daily violence has dealt devastating blows to the quality of life for average Iraqis: health care services are “stretched to the limit”, food shortages have led to an increase in malnutrition and basic infrastructure for electricity and sanitation have not worked for years in many areas.

“The conflict in Iraq is inflicting immense suffering on the entire population,” states the ICRC report entitled Civilians Without Protection.

“Civilians bear the brunt of the relentless violence and the extremely poor security conditions are disrupting the lives and livelihoods of millions. Much of Iraq’s vital infrastructure is in a poor state of repair, owing to lack of maintenance and security constraints have impeded repair work on electrical power grids, water and sanitation systems, medical facilities and other essential facilities.”