The treatment of Rohingya Muslims is “besmirching” Myanmar’s reputation, British foreign secretary Boris Johnson warned Aung San Suu Kyi.
Mr Johnson’s comments come on the back of escalating violence in the Rakhine state, with graphic reports emerging of young children being beheaded and others being burned alive.
Around 400 Rohingya are thought to have died in the latest killing spree, which erupted over a week ago.
Soldiers and armed residents are accused of burning houses and committing atrocities throughout the Rakhine state. South East Asian human rights organisation Fortify Rights said thousands of Rohingya civilians have been left homeless as the fires spread through whole villages.
According to the UN’s refugee agency, some 58,000 people have fled into neighbouring Bangladesh as they seek to escape the bloodshed. Some have drowned while attempting to make the journey.
In a statement, Mr Johnson said: "Aung Sang Suu Kyi is rightly regarded as one of the most inspiring figures of our age but the treatment of the Rohingya is alas besmirching the reputation of Burma.
"She faces huge challenges in modernising her country... It is vital that she receives the support of the Burmese military, and that her attempts at peacemaking are not frustrated.
"She and all in Burma will have our full support in this."
He added that he was hopeful Ms Suu Kyi would be able to use "all her remarkable qualities to unite her country, to stop the violence and to end the prejudice that afflicts both Muslims and other communities in Rakhine".
Ms Suu Kyi, who won the Nobel Peace Prize for her pro-democracy activism, is considered Myanmar’s de facto leader.
She has been criticised for not doing enough to defend the one-million-strong Rohingya community, which has been persecuted and considered stateless since 1982, despite having lived in Myanmar for generations. Myanmar, a Buddhist-majority country, classifies members of the group as "Bengali" and, therefore, belonging in Bangladesh.
UN secretary general Antonio Guterres has warned of a looming “humanitarian catastrophe” in western Myanmar, while Turkish president Recep Tayyip Erdogan has accused the country of “genocide” against the Rohingya.
Human rights group Burma Campaign UK said that Boris Johnson’s comments could have gone further.
The group said that the British government needs to be pressurising Ming Aung Hlaing, the armed forces’ commander-in-chief, in order to halt the attacks.
Its director, Mark Farmaner, also said the Foreign Secretary should have explicitly referred to human rights violations. In a tweet, Mr Farmaner said: “Why no mention of mass killings, burning villages or use of the words human rights?”