LASHIO, Myanmar - Nearly 90,000 civilians in northeastern Myanmar are thought to have fled clashes between ethnic rebels and the nation's army, an official said Wednesday, as sporadic violence hampered efforts to evacuate those still trapped in the conflict zone.

Rebels fear a major assault from the national army is being planned, following the imposition of a state of emergency in the Kokang region of Shan state on Tuesday.

Whole towns and villages lie empty in the rugged, remote area as tens of thousands of residents have fled their homes - some on foot - with one local MP suggesting over half of the local population was now on the move.

At least 30,000 people, mainly the ethnically Chinese Kokang, have crossed the border into Yunnan state - sparking alarm in Beijing.

Clashes continued Wednesday as more civilians arrived in the Shan town of Lashio fearing they could be caught up in the sudden convulsion of violence.

In one of at least two separate attacks on civilians on Tuesday, around a hundred people came under fire as they travelled in Myanmar Red Cross trucks in a desperate dash from their homes.

"It was a miracle we weren't hit," Maung Ying told AFP in Lashio after Tuesday's attack on aid vehicles, which were marked with Red Cross flags but had no military protection.

"They were shooting from the mountains on both sides of the road. I thought I was going to die, bullets were passing just over our heads," he said, adding the ordeal in broad daylight lasted for an hour.

The convoy had passed through Laukkai - the epicentre for fierce fighting since rebels launched attacks last week - and which is now a no-go area for the Myanmar Red Cross.

Tun Tun Oo, head of the Lashio Red Cross, which is separate from the better known International Committee of the Red Cross, said it was now too dangerous to go into Laukkai.

"We just have to wait for people to come to us. If the army could give us protection under military rule, then we would go back," he told AFP. Both the army and rebels have blamed each other for the attack.

The violence, which flared on February 9, has killed dozens of soldiers and rebels. There is no official civilian death toll.

Laukkai's ethnic Kokang MP Kyaw Ni Naing said he believed more than half of the region's 140,000 population were on the move.

"There are many refugees fleeing from Laukkai town and nearby regions," he told AFP by telephone from the capital Naypyidaw, adding that sporadic fighting had continued on Wednesday, although the town itself was quiet.

He said around 8,000 people from central Myanmar had fled through Lashio, while some 40,000 Kokang had crossed into China. Another 40,000 people from Kokang and other Myanmar ethnic minorities were thought to be scrambling for safety along the frontier.

On the Chinese side, Beijing says it has stepped up border controls and called on all parties to prevent a further escalation of fighting.

Military activity in Lashio appeared to be increasing on Wednesday, despite an apparent lull in heavy fighting, with two helicopters and six trucks seemingly on standby at an army airstrip on the outskirts of town.

"The military is increasing its forces with tanks and heavy weapons in the region. We believe there will be more fighting," said Captain Tar Parn Hla, spokesman for the Ta'ang National Liberation Army, which is fighting alongside the Kokang rebels.

He denied rebel involvement in the attack on the aid convoy, and accused the army of "intentionally" targeting civilians.

No one from the Myanmar government was available to comment.

It is the first major unrest in the region since 2009, when a huge assault by the army drove out rebel fighters, sending tens of thousands into China.

Myanmar's quasi-civilian government has made signing a nationwide ceasefire a key pillar of its reforms as the country heads towards a general election later this year.

But the fighting has raised fears those efforts are unravelling.

The army, which for decades used the civil conflicts in ethnic regions as a justification for its iron-fisted rule, has sought to portray the current fighting as a "just war" in state media.