A senior U.N. aid official said on Tuesday she had witnessed a level of human suffering never seen before in camps for some 140,000 stateless Muslim Rohingya in Myanmar's Rakhine state, displaced by Buddhist-Muslim violence sparked in 2012.

Deputy Emergency Relief Coordinator Kyung-Wha Kang visited Myanmar last week, travelling to western Rakhine state and northern Kachin state, where more than 100,000 people have been displaced since fighting between ethnic minority insurgents and the government erupted in June 2011, ending a 17-year ceasefire.

"In Rakhine, I witnessed a level of human suffering in IDP (internally displaced person) camps that I have personally never seen before, with men, women, and children living in appalling conditions with severe restrictions on their freedom of movement, both in camps and isolated villages," Kang said.

"Many people have wholly inadequate access to basic services including health, education, water and sanitation," she said.

Kang added that the safety and security of U.N. staff and other aid workers also remained at risk until the Myanmar authorities ensured the perpetrators of attacks against NGO and U.N. offices in March were brought to justice.

In February Myanmar authorities expelled Medecins Sans Frontieres - the main aid group in Rakhine - and then the March attacks led to the withdrawal of more aid groups, sparking a health crisis in the camps that the government has dismissed.

"Humanitarian workers in Rakhine are carrying out their work under extremely difficult circumstances and I was humbled by their commitment to stay and deliver," Kang said.

Myanmar describes the Rohingya as Bengalis, a term that implies they are illegal immigrants from neighbouring Bangladesh. Many say they have lived in Rakhine for generations.

In Kachin, Kang said she was only able to visit one camp in a government-controlled area, but also spoke with aid workers helping in areas held by the Kachin Independence Army rebels.

"Access by international humanitarian organizations is improving through cross-line missions but aid agencies need regular, predictable, and sustained access to all IDPs," she told a news conference in New York.

The United Nations has warned the violence in Rakhine state poses a serious threat to the country's dramatic economic and political reforms as it emerges from half a century of military rule.

Myanmar President Thein Sein heads a quasi-civilian government installed in 2011 after Myanmar's military stepped aside. Under his reforms, opposition leader and Nobel peace prize laureate Aung San Suu Kyi, who spent 15 years under house arrest, has been allowed back into politics and has made a number of visits abroad.