The chief of the United Nations humanitarian relief agency has urged Myanmar's ruling military to allow greater access to 18 million people in need of aid, describing the situation as critical as a post-coup conflict intensifies.

Returning from a three-day trip that included a meeting with top general Min Aung Hlaing, Martin Griffiths said a funding shortage was also complicating efforts to reach the third of Myanmar's population that was in need of assistance.

Myanmar has been locked in crisis since the military wrested back control after a decade of unprecedented reform under quasi-civilian governments.

The military's bloody crackdown on dissent led to the formation of an armed resistance movement that has battled security forces around the country, with clashes displacing more than a million people.

"Successive crises in Myanmar have left one third of the population in need of humanitarian aid," Griffiths said in a statement.

"They expect more and better from their leaders and from the international community."

The UN agency said fighting and natural disasters since the 2021 coup had led to a five-fold increase in the number of displaced people, from 380,000 to 1.9 million.

Griffiths said humanitarian relief organizations were struggling with insufficient resources and urged international donors to do more, with just 22% of the annual funding requirements received by mid-year.

He said he pressed the junta to expand access and expressed concern about civilians and restrictions and bureaucracy preventing aid groups from helping them.

The junta has a testy relationship with the UN after numerous investigations that have accused the military of atrocities against civilians, which it has rejected.

The UN Human Rights report in June said the lack of aid access may amount to war crimes, while a team of UN investigators last week said war crimes were "increasingly frequent and brazen."

State media reports on the visit said the international community "should seek accurate information on Myanmar's situation."