Meanwhile, the United Nations appealed on Thursday for massive help for almost 400,000 Rohingyas who have fled to Bangladesh, with concern growing that the number could keep rising, unless Myanmar ends what critics denounce as "ethnic cleansing".
When asked why the US government hasn't condemned the government of Myanmar yet, Nauert said there are reports of extrajudicial killings, sexual violence, the burning Rohingya villages by security forces and also non-Rohingya civilians and her department has condemned the violence at all levels and it couldn't be stronger than that.
During a press conference at Lancaster house in London on Thursday, Tillerson stressed that the violence against the Rohingya, a stateless Muslim minority group, must stop.More news: Knife-wielding man 'attacks French soldier in Paris'
US Deputy Assistant Secretary of State Patrick Murphy is expected to travel to Myanmar in the coming days for discussions on the crisis, after summoning Myanmar's ambassador to express US concerns on Wednesday.
The Arakan Rohingya Salvation Army (ARSA) said it is trying to defend the minority group from a long campaign of persecution and violence.
"I think it is important that the global community speak out in support of what we all know the expectation is for the treatment of people regardless of their ethnicity", he added. "I think many people around the world share that admiration". Since August alone, more than 270,000 Rohingya families have fled across the border to Muslim-majority Bangladesh.More news: Pradyuman murder case: Haryana govt orders CBI probe
"Violence in Myanmar has created a humanitarian catastrophe".
Britain and the United States have issued their clearest call yet for the Myanmar Government to step in and stop what the United Nations calls an "ethnic cleansing" operation in the country's west.
According to the government of Bangladesh - where thousands of Rohingya have fled - around 3,000 Rohingya have been killed in the crackdown.More news: Oil prices rise as IEA sees higher demand, shrinking inventories
The Rohingya are fleeing from a Myanmar military offensive in Rakhine that began after a series of guerrilla attacks on August 25 on security posts and an army camp.