US military authorities revealed that thousands more National Guard troops have been assigned to duty in Iraq in deployments scheduled for the next three years. The announcement coincided with reports that twelve more US and British troops were killed over a three-day period in the war-torn country.

Both the spike in casualties and the announcement that National Guard units will be sent back for second tours of duty underscore the increasingly precarious state of the US military occupation of Iraq. Even the most slavish US ally, the British government of Prime Minister Tony Blair, is reducing rather than increasing its forces there.

Eight American and four British soldiers were killed in incidents Tuesday, Wednesday and Thursday. Seven of the American soldiers were killed in and around Baghdad, with the other death taking place in Diyala province, an area of mixed Sunni, Shiite and Kurdish population north of the capital. The American death toll has hit 18 in only the first six days of April.

The overall US death toll in Iraq is approaching 3,300, but individual units have been hit far harder than that number might suggest. A lengthy report of conditions in Diyala province, published April 2 in the New York Times, profiled the 1-12 Combined Arms Battalion, an American combat unit of nearly 1,000 men that has suffered 21 deaths and 93 wounded since arriving in the city of Baquba in November. This works out to a casualty rate of more than 10 percent in five months, less than half the unit’s scheduled time of deployment.

Also this week, a US helicopter crashed near the town of Latifiya, 20 miles south of Baghdad, a stronghold of Sunni-led guerilla resistance to the occupation. Four US soldiers were injured but none were killed. US officials initially denied that the helicopter had been shot down, saying mechanical failure was to blame, although such claims have proven inaccurate in the past.

It was the ninth US helicopter shot down or crashed since January 20, a sharp escalation over previous years and an indication of either improved weaponry or growing proficiency on the part of the insurgency. At that rate, the occupation forces would lose between 40 and 50 helicopters this year.

Three security stations in Baghdad neighborhoods, manned by mixed Iraqi and American units, were attacked Thursday in what may have been coordinated actions. A car bomb exploded at the station in Khadra, while mortar shells hit stations in Sadr City and Mansour.

The security stations are a keystone of the “surge” strategy being implemented by the new US commander, General David Petraeus, in which American troops are leaving their large, protected bases and deploying to more vulnerable posts in heavily populated neighborhoods of the capital city.

An Iraqi Army post near a rural prison in the village of Zinzala, 20 miles west of Mosul, was overrun by insurgents and its garrison of 10 soldiers wiped out, according to Iraqi officials.

The four British soldiers, two of them women, were killed in a single incident, when a massive roadside bomb blew up a British armored vehicle. An interpreter died along with the soldiers. The bombing came in the course of combat between the British troops and a Shiite militia force in the Hayaniyah district west of the southern Iraqi city of Basra.

The two female soldiers were from intelligence and medical units. The British patrol was said to be engaged in an intelligence-led operation searching for a weapons cache. No weapons were found, and the unit was ambushed by insurgents firing small arms and rocket-propelled grenades. As the troops sought to retreat to Basra they struck the improvised explosive device, which left a huge crater in the road, destroying one armored car and damaging another.

Photographs of the incident showed Iraqis celebrating the destruction of the British armored vehicle, with a man holding up a British military camouflage helmet, other men waving and smiling, and a child brandishing a piece of charred metal from the wreckage.

Two more British soldiers were killed April 1-2, bringing the total deaths for the week to six, an exceptionally high number for a force which has lost a total of 140 dead in four years of war and occupation.

A Pentagon official who was not identified by the media made the announcement Friday that four National Guard brigades, each with about 3,500 soldiers, had been identified for deployment to Iraq between January 2008 and 2010.

The deployments will violate a longstanding policy under which National Guard troops, a key reserve component of the military, spend five years at home for every year of mobilization. This policy has effectively blocked the sending of National Guard troops for second tours of duty in Iraq, while many units of the regular Army are on their third and even fourth tours.

Defense Secretary Robert Gates has not yet formally signed the order designating which National Guard units would take places in the regular rotation to replace Army units returning from Iraq. Press reports suggested that brigades from North Carolina, Florida, Arkansas and Indiana were among those that might be affected.

The Guard troops would not be part of the expanded deployment of US forces focused on Baghdad, the ongoing “surge” that Bush announced in January. They would not likely be dispatched overseas until December of this year. About three quarters of the Army National Guard—270,000 out of 350,000—have served either in Afghanistan or Iraq since those wars began.

Gates said Thursday that the Pentagon still adhered to the policy of one year on, five years off for National Guard units, but he told reporters there would be a “transition period during which those guidelines would be violated and in which we would be unable, because of the troops commitments in Afghanistan and in Iraq, to meet those goals.”

Earlier in the week, the Pentagon announced it would cut short the stateside assignment of two military units and send them back to Iraq after less than a year at home. The troops affected were from the headquarters unit of the 4th Infantry Division, at Fort Hood, Texas, and the 1st Brigade Combat Team of the 10th Mountain Division from Fort Drum, New York.

While this action violated a stated policy of giving regular units two years at home for every year in Iraq, a Pentagon official echoed Gates’s remark about the National Guard, saying, “When we are a nation at war we might not always be able to make those policy goals that we have for ourselves.”

Two other units had their stays in Iraq extended. The 2nd Brigade Combat Team of the 82nd Airborne Division, sent into Baghdad as part of the surge, will remain in Iraq a full year, up from the nine months originally announced. The headquarters unit of the 25th Infantry Division, scheduled to return to Hawaii in July, will stay until September.

Gates commented Friday about the growing uncertainty over the progress of the surge in Baghdad, saying it would be at least until the middle of the summer before commanders could evaluate its outcome. He backed away from previous comments that troops deployed in the surge could return from Iraq by Christmas, not disputing the suggestion by Lt. Gen. Raymond T. Odierno, Petraeus’ top deputy, that US forces would have to sustain higher troop levels until at least early in 2008.