YANGON: The UN's humanitarian chief has called on Myanmar to stop blocking aid to thousands of people displaced by conflict with northern ethnic rebels, raising concerns over welfare as winter draws in.

The United Nations has been prevented from reaching more than half of the 75,000 people forced from their homes by fighting between the army and minority Kachin insurgents, said Valerie Amos in a statement sent overnight after a visit to the area on Thursday.

“The UN has not been allowed access to provide badly needed assistance to some 39,000 people in areas outside the government's control since July 2012,”said the under-secretary-general for humanitarian affairs and emergency relief coordinator.

She said local partners were supplying food and other relief, “but their stocks are depleted and with the winter months approaching, getting more supplies in is critical”.

Fierce fighting has raged in Myanmar's Kachin state since a 17-year ceasefire between the military and the Kachin Independence Army (KIA) shattered in June 2011.

A spokeswoman for the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) told AFP the government had stopped access because it “cannot guarantee our security so it hasn't granted us permission to take a convoy to the rebel areas”.

The UN envoy met President Thein Sein, who has overseen sweeping reforms since taking power at the helm of a quasi-civilian government last year, and raised fears over lack of access to both government and rebel controlled camps in Kachin, the statement said.

Myanmar, which has been racked by pockets of civil war in its ethnic areas since independence in 1948, has signed tentative ceasefire deals with a number of major armed groups. But a deal with Kachin rebels remains elusive, with deep distrust on both sides.

Continued fighting in the region, along with the eruption of communal violence between Buddhists and Muslims in western Rakhine state, is seen as a key stumbling block on Myanmar's reform path.

Amos, who has visited both regions during her four day trip to Myanmar, has described the conditions in camps for stateless Rohingya Muslims in Rakhine as “dire” and expressed concern over the plight of some 115,000 people displaced in that state.