Thousands of people fled heavy fighting on Myanmar's northern border with China overnight, activists said Wednesday, as the government blocked a senior UN official from visiting the area.

Clashes between the army and ethnic minority militias in Myanmar's borderlands have intensified in recent months, undercutting Aung San Suu Kyi's vow to bring peace to the country since her party took power in March.

Dozens have been killed and thousands displaced since fresh fighting erupted between the Myanmar military and the ethnic minority Kachin Independence Army (KIA) in November.

The unrest has rippled across Shan and Kachin states, threatening the next round of peace talks between the government, military and ethnic groups scheduled for February.

Dashi Naw Lawn, secretary of the Kachin Network Development Foundation, said the army launched airstrikes near the border town of Laiza on Tuesday.

"There was a big fight yesterday and the Myanmar army used planes to attack the area around Laiza," he told AFP on Wednesday.

"The fighting is getting worse and worse."

Kachin activist Khon Ja said some 3,600 people had fled from two IDP camps overnight to escape the violence.

She cited testimonies from escapees saying around a third had crossed a river into China, while the rest were stranded on the Myanmar side.

The UN's relief agency said this week that 2,700 displaced people had been moved to new camps because of the clashes, warning there was not enough shelter to protect them against dropping temperatures.

Myanmar's army said its troops have killed seven KIA rebels and seized several bases over the weekend.

The growing crisis comes as UN special rapporteur Yanghee Lee was barred from visiting the conflict area this week as part of a 12-day trip to probe escalating violence in the country.

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

On Friday she is expected to visit Rakhine state to probe claims the army has been carrying out severe human rights abuses against the Muslim Rohingya minority -- allegations the government has denied.