"Any adverse event for a veteran within our care is one too many," Shinseki said at a Senate hearing Thursday on the Phoenix allegations and other problems at the VA. "We can, and we must do better."
Meanwhile, Sen. Patty Murray, D-Wash., said the hearing "needs to be a wake-up call for the department," noting that outside reviews have outlined problems with wait times and quality of care since at least 2000.
Shinseki's testimony marked his first extended comments since allegations surfaced last month that the Phoenix VA hospital maintained a secret waiting list to hide lengthy delays for sick veterans. A former clinic director says up to 40 veterans may have died while awaiting treatment at the Phoenix facility.
Some Republicans and veterans groups have called for Shinseki to resign.
Pushing back against sometimes hostile questions, Shinseki said he welcomes a White House review of his beleaguered department after allegations the Phoenix hospital maintained a secret waiting list to hide lengthy delays for sick veterans. A former clinic director says up to 40 veterans may have died while awaiting treatment at the Phoenix facility.
"If allegations about manipulation of appointment scheduling are true, they are completely unacceptable - to veterans, to me and to our dedicated VA employees," Shinseki said.
The hearing before the Senate Veterans Affairs Committee comes as President Barack Obama has assigned White House deputy chief of staff Rob Nabors to work on a review focused on policies for patient safety rules and the scheduling of patient appointments. The move, announced late Wednesday, signals Obama's growing concern over problems at the VA. Problems similar to those that surfaced in Phoenix have since been reported in other states.
Sen. Patty Murray, D-Wash., said at the hearing, "It's extremely disappointing that the department has repeatedly failed to address wait times for health care," Murray said.
Murray told Shinseki she believes he takes the allegations seriously and wants to do the right thing, "but we have come to the point where we need more than good intentions."
Murray called for Shinseki to take "decisive action to restore veterans' confidence in VA, create a culture of transparency and accountability and to change these system-wide, years-long problems."
Sen. John McCain, R-Ariz., said the Obama administration "has failed to respond in an effective manner" to allegations made public more than a month ago. "This has created in our veterans' community a crisis of confidence toward the VA - the very agency that was established to care for them," McCain said.
"Treating those to whom we owe the most so callously - so ungratefully - is unconscionable and we should all be ashamed," McCain said.
The American Legion and some congressional Republicans have called for Shinseki to resign, a move he and the White House have resisted. The VA's inspector general is investigating the Phoenix claims, and Shinseki has ordered an audit of VA facilities nationwide to see how they provide access to care.
A White House official said Shinseki requested more help with the review, leading Obama's chief of staff, Denis McDonough, to tap Nabors for the assignment. Shinseki said he welcomed Nabors' help in making sure veterans receive high-caliber health care in a timely fashion.
"While we get to the bottom of what happened in Phoenix, it's clear the VA needs to do more to ensure quality care for our veterans," Obama said in a statement.
The chairman of the Senate committee said there were "serious problems" at the VA but lawmakers must avoid a rush to judgment.
"I don't want to see the VA system undermined," Sen. Bernie Sanders, I-Vt., told The Associated Press. "I want to see it improved. I want these problems addressed."
"If people are cooking the books, running second books, that is wrong. That's illegal and we have to deal with it," Sanders said, adding he also was troubled by reports that some veterans have to wait up to six months to see a doctor.
The VA system is the largest health care system in the country, serving nearly 9 million veterans a year at 152 hospitals and more than 1,500 other sites nationwide. Surveys show patients are mostly satisfied with their care. But with such a huge system, "there are going to be problems," Sanders said.
The Phoenix VA Health Care System, which includes a hospital and at least a half-dozen satellite clinics, serves about 80,000 veterans. "It's huge," Sanders said. "Do we have enough doctors and nurses and nurse practitioners at that site? If not, why not?"
Similar questions can be asked about VA sites across the country, Sanders said, calling quality of care an issue "the VA struggles with every day" at its hospitals and clinics nationwide.
"We have a moral obligation to take care of these veterans," he said. "We can do better. We must do better."
The House Veterans Affairs Committee voted last week to subpoena all emails and other records in which Shinseki and other VA officials may have discussed destruction of what the committee called "an alternate or interim waitlist" for veterans seeking care in Phoenix.
A top VA official had told congressional staff last month that the "secret list" referred to in news reports may have been an "interim list" created by the hospital.
Shinseki, a retired four-star Army general, "has done the right thing" by calling for an immediate investigation by the inspector general's office, Sanders said, adding it was premature to call for Shinseki to step down.
"While it might be temporarily satisfying to call for firing someone, it doesn't get us any closer to the truth or solve problems that may exist," Sanders said.
AP White House Correspondent Julie Pace contributed to this report.