21/12/2016: Correction: An earlier version of this article referred people drowning in a river between Myanmar and Malaysia. Those fleeing drowned trying to reach Malaysia from Myanmar via Thailand.
As many as 15,000 people have fled across Myanmar's border into China in the past month as fighting between the army and armed ethnic groups intensifies, the UN says.
Aid access to people affected by conflict in the northern states of Kachin and Shan "is getting worse, not better", Pierre Peron, a spokesman for the UN Office for the Coordination of Humanitarian Affairs (OCHA) in Myanmar, said by email.
| Is the world ignoring the plight of the Rohingya?
The OCHA said in an update on Monday that, as well as the estimated 15,000 new refugees, another 2,400 people have been displaced internally in the northern part of Shan state since November 20, when a coalition of four rebel armies attacked military and police outposts.
Dozens have also drowned trying to cross the river that separates Myanmar from Thailand in an attempt to reach Malaysia. The government of Myanmar has already banned its citizens from working in Malaysia.
Weeks of clashes and the new displacements have damaged Myanmar leader Aung San Suu Kyi's hopes of securing peace in the long-running conflicts in the mountainous border areas, a goal she has made her administration's top priority.
Observers fear that Suu Kyi's fledgling civilian administration is unable to rein in the army, which retains political power and is free from civilian oversight.
Critics have accused her of turning a blind eye to the plight of Rohingya ethnic minority, who have faced discrimination from the authorities for decades.
Echoing a November 30 UN statement, Amnesty International on Sunday said abuse of the ethnic minorities based mostly in western Rakhine state might "amount to crimes against humanity".
"Humanitarian access to conflict areas in Kachin and Shan states is currently worse than at any point in the past few years," Peron said.
"This has seriously affected the ability of humanitarian organisations to provide life-saving aid to tens of thousands of [internally displaced] and other conflict-affected people."
READ MORE: Rohingya refugees from Myanmar tell of trauma
The army said it would step up operations in Shan state following the November 20 attacks.
A Myanmar police official, who requested anonymity because he was not authorised to speak to reporters, told Reuters news agency that government forces had clashed with armed groups in northern Shan at least 170 times in the past month.
In a separate offensive to the north, government forces took control of a strategic hill close to the Kachin Independence Army's headquarters at Laiza on the Chinese border on Saturday, according to state media.
Shells reportedly landed near a camp for the internally displaced just outside Laiza on Sunday, the OCHA said, citing unconfirmed reports that it could not verify independently.
No casualties were reported but shelters were damaged and about 400 people had to be evacuated, it said.
A Myanmar government spokesman was not immediately available for comment.
Meanwhile, at least 30,000 have been internally displaced in Rakhine state, where fighting and crackdowns have been severe since nine border police officers were killed near Myanmar's border with Bangladesh.
While thousands of Rohingya have found shelter in Bangladesh, the country is now turning away further arrivals of boatloads of fleeing families.
Human Rights Watch has released several sets of satellite images showing that several Rohingya villages in Rakhine state have been burned and destroyed since late October.
Source: News agencies