Cliff Owen/AP Photo

As Congress works on a long-term plan to reform the Veterans Administration, first lady Michelle Obama used her platform to speak out on ending veterans' homelessness.

Obama called the more than 58,000 homeless American veterans a "stain on the soul of this nation."

"As Americans, the idea that anyone who has worn our country's uniform is sleeping on the ground should horrify us," Obama told an estimated 1,600 federal and local officials and homeless advocates in Washington, D.C., at the 2014 National Conference on Ending Homelessness.

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"That moral and patriotic duty is only part of the reason why ending veteran homelessness is so critical. As we all know, ending homelessness for our veterans can also be a crucial first step, a proof point to show that we can end homelessness for everyone in this country."

Obama called the work of homeless advocates "critically important."

"Day after day, as you fight for more resources, you encounter too many folks who don't take you seriously because they don't believe that we will ever, truly be able to solve this problem," she said. "Yet when so many others accept homelessness as a fact of life, you refuse to give up.

"I just want to say thank you."

Obama has made ending veterans' homelessness a priority during her tenure as first lady, calling it a "moral outrage" at a White House event last month to launch the Mayors' Challenge to End Veteran Homelessness.

Since June, 182 local leaders have taken the challenge to end veteran homelessness in their communities by the end of 2015.

"We are seeing that with enough resources and the right strategies - strategies like housing first, rapid re-housing - we can make huge amounts of progress in a very short period of time," Obama said.

In an op-ed article published Wednesday, Obama pointed to success in Phoenix and Salt Lake City.

"Any number of veterans left out in the cold is too many, but those numbers show us that even in some of our largest metropolitan areas, ending veteran homelessness is eminently achievable," she wrote.