But he said the department plans to temporarily cease the use of mandatory overtime, an initiative credited as an important part of the reduction.
Shinseki said the agency was easing back on overtime, which began in May, to avoid exhausting the workforce.
"You can only run full throttle for so long," he said during a news media roundtable at VA headquarters in Washington.
Shinseki also credited the launch of the VA's new digital claim system as a key reason for the decline.
"We have been standing astride a river of paper for decades," he said.
Progress on the backlog slowed during the partial government shutdown, when the VA was forced to stop mandatory overtime.
"We took a 17-day pause and watched a flat line," Shinseki said.
The overtime was reinstated when the shutdown ended, but only until Nov. 23, when it will temporarily cease again until late January.
Shinseki said the agency will monitor what effect halting the overtime again for two months has on continued progress.
The VA would continue overtime use "with a fine hand on the tiller," he said. "We're sensitive to the concerns of a workforce that we're already asking a lot of."
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