YANGON, (Reuters) - Fighting between Myanmar forces and a rebel group in the remote northeast could escalate, a spokesman of the Kachin Independence Army (KIA) told Reuters on Friday, worsening a humanitarian crisis that has displaced more than 4,000 people.

One of Myanmar’s most powerful rebel groups, the KIA has regularly clashed with government troops in the mountainous region bordering China and India since 2011, when a 17-year-old ceasefire broke down.

Since early April, the conflict has escalated, driving more than 4,000 people from their homes, civil society groups based in Kachin have said.

Clashes have been reported in half of the 18 townships in the region, where the Myanmar military has sent about 2,000 infantry troops, fighter aircraft and helicopters to combat KIA forces, said the spokesman, Col Naw Bu.

“The army is sending more troops from the lower part of Myanmar and that’s why the fighting will be more intense,” he said.

“The number of refugees could increase and they could suffer more challenges,” he added, calling the clashes “the worst conflict” since Kachin soldiers started fighting the government in the early 1960s.

He declined to give details of the casualties sustained by the KIA or its military deployment.

Myanmar’s military and civilian government officials did not immediately respond to requests from Reuters for comment.

Aid groups have urged the government to allow access to the village of Aung Lawt, where about 2,000 people have been trapped in a remote forest with no access to humanitarian aid for two weeks.

“The longer it takes, the more challenging it is for them. And most of the people are elderly, women and children,” said Gum Sha Aung, an official of a coalition of Kachin-based humanitarian groups.

The United States embassy in Yangon said it was “deeply concerned” about intensified fighting in Kachin.

In a Twitter message on Friday, it urged the government and military to “protect civilian populations and allow humanitarian assistance” to those affected by the recent conflicts.

A coalition of 32 groups in Kachin this week urged the United Nations Security Council to refer Myanmar to the international criminal court, citing recent deaths and displacement of civilians in Myanmar army operations.

“Women, children, elderly people forced to flee their homes because of the relentless fighting in Kachin,” Mark Cutts, the head of a U.N. agency coordinating humanitarian affairs in Myanmar, said in a Twitter message on Thursday.

“Local humanitarian organizations doing outstanding work trying to help. But when will this decades-old conflict end?”

Reaching a comprehensive peace deal with Myanmar’s ethnic minorities has been leader Aung San Suu Kyi’s stated top priority, but the Buddhist-majority country has seen the worst fighting with rebels in years since she took office in 2016.

Ending the civil war, eclipsed in media coverage by the plight of nearly 700,000 Rohingya Muslim refugees who fled to neighboring Bangladesh, is key to further development of the resource-rich country.