U.S. Secretary of State Rex Tillerson said Wednesday that the United States is concerned about "credible" reports of atrocities committed by Myanmar security forces against Rohingya Muslims, causing more than 600,000 of them to flee to Bangladesh in recent months, and seeks an "independent investigation."

"We're deeply concerned by credible reports of widespread atrocities committed by Myanmar's security forces and by vigilantes who were unrestrained by the security forces during the recent violence in Rakhine State," Tillerson told a joint press conference after talks with Myanmar's leader Aung San Suu Kyi in the capital Naypyitaw.

"We're also distressed by the fact that hundreds of thousands of men, women and children have been forced to flee to Bangladesh," he said, calling the scale of the humanitarian crisis "staggering."

At the same time, Tillerson condemned the Aug. 25 attacks by Rohingya militants on Myanmar security forces that initiated the latest round of violence in the western state of Rakhine.

Rohingya refugees and international human rights organizations claim the security forces' response has involved extreme violence, rape and the razing of entire villages, in what a top U.N. human rights official, Zeid Ra'ad Al Hussein, has said appears to be a "textbook example" of ethnic cleansing.

Tillerson declined to endorse that view, saying, "Clearly, what we know occurred in Rakhine state...has a number of characteristics of certainly crimes against humanity. Whether it meets all the criteria for ethnic cleansing, I think we continue to evaluate that ourselves."

"The recent serious allegations of abuses in Rakhine State demand a credible and impartial investigation," he said, while also calling for it to be "full and effective" and "independent," with full cooperation from the military.

In the meantime, he said, the United States "will continue to work with our partners to assure there are consequences for individuals confirmed to have been responsible for atrocities using all available mechanisms including those available under U.S. law."

"Those who commit human rights abuses or violations must be held accountable," he added.

Suu Kyi thanked Tillerson for approaching the complex issue with an open mind and said they "agreed that it is most important we should bring peace and stability to this country, and that can only be done on the basis of rule of law."

Asked why she has been silent on the plight of the Rohingya, the Nobel Peace Prize laureate said, "I haven't been silent...but I think what people mean is that what I say is not interesting enough."

"But what I say is not meant to be exciting, it's meant to be accurate and it's aimed at creating more harmony and a better future for everybody, not for certain people against each other."

The Myanmar government has long claimed that the Rohingya, a stateless Muslim community in the overwhelmingly Buddhist nation, are illegal immigrants from Bangladesh or the state of West Bengal in eastern India, and not legitimate citizens of Myanmar, even though many have lived there for generations.

Tillerson called for a "transparent and fully voluntary path to citizenship" for Rohingya Muslims, one that that is "considered credible by all stakeholders."

"Myanmar's response to this crisis is critical to determining the success of its transition to a more democratic society," he said. "A key test of the new democracy is how it treats its most vulnerable and marginalized populations."

Tillerson, who arrived earlier in the day for a one-day visit, also met with the head of the military, Commander-in-Chief Senior Gen. Min Aung Hlaing.

He announced that the United States will provide an additional $47 million for humanitarian assistance to address the "extremely dire" Rohingya refugee situation, bringing total U.S. contributions since August to $87 million.