Heavy fighting in Myanmar’s northern state of Kachin has forced thousands of people to flee their homes and prevented a United Nations (UN) special envoy from visiting the embattled area near the Chinese border, activists say.

According to the secretary of the Kachin Network Development Foundation, Dashi Naw Lawn, army warplanes carried out air raids near the town of Laiza on Tuesday, AFP reported Wednesday.

“There was a big fight yesterday and the Myanmar army used planes to attack the area around Laiza,” he said. “The fighting is getting worse and worse.”

Activist Khon Ja said the airstrikes forced some 3,600 people to escape from two camps for internally displaced persons overnight.

Myanmarese people cross a river near the China-Myanmar border to take refuge in China, November 30, 2016. (Photo by AFP)

Around a third of the escapees had managed to cross a river into China, but the rest had been stranded on the Myanmar side, according to Khon.

The fighting intensified as UN special rapporteur Yanghee Lee was scheduled to visit the area on a 12-day visit to the country.

“She was not allowed to travel to Hpakant and Laiza,” said her spokesman Aye Win.

The northern state of Kachin has been the scene of fighting between the military and autonomy-seeking ethnic rebels for decades. The battle has forced more than 100,000 people out of their homes over the years.

The army said troops had killed seven rebels and seized several bases over the weekend.