That is more than one-third of the total who were told to report to a mobilisation station by 17 October.
Three weeks ago, the number stood at 622 amid talk that any who refused to report for duty could be declared absent without leave. Refusing to report for duty normally would lead to AWOL charges, but the US army is going out of its way to resolve these cases as quietly as possible.
In all, 4166 members of the Individual Ready Reserve have received mobilisation orders since 6 July, of which 2288 were to have reported by 17 October. The others are to report in coming weeks and months.
Of those due to have reported by now, 1445 have done so, but 843 have neither reported nor asked for a delay or exemption. That no-show rate of 37% is roughly in line with the one-third rate the US army had forecast when it began the mobilisation to fill positions in regular and reserve units.
Members of the Individual Ready Reserve, or IRR, are rarely called to active duty. The last time was 1990, when nearly 20,000 were mobilised.
The first IIR Marine death in Iraq
IRR members are people who were honourably discharged after finishing their active-duty tours, usually four to six years, but remained in the IRR for the rest of the eight-year commitment they made when they joined the army.
The Marine Corps, meanwhile, said on Friday that a marine killed in western Iraq earlier this week, Sergeant Douglas Bascom, 25, was a member of the Individual Ready Reserve.
He was the first IRR marine to die in Iraq, according to Gunnery Sergeant Kristine Scharber, a spokeswoman at Marine Corps headquarters in the Pentagon.
There are about 400 IRR marines deployed in Iraq and Afghanistan, according to Shane Darbonne, a spokesman for the Marine Corps Mobilisation Command.
Army officials said they were uncertain whether any of their Individual Ready Reserve members had been killed in Iraq.
That the US army has had to reach so deeply into its store of reserve soldiers is a measure of the strain the Iraq and Afghanistan campaigns have put on the active-duty US army.
When the American invading force toppled the Iraqi government in April 2003, the US army thought it would be sending most of its soldiers home within months. Instead, it has kept 100,000 or more there ever since.