North Dakota governor Jack Dalrymple ordered the immediate evacuation of the main encampment of activists opposing the Dakota Access pipeline, but a state official said he would not authorize law enforcement officials to carry out the order.

Citing concerns about “harsh winter conditions” and the North Dakota Disaster Act of 1985, Dalrymple said on Monday that all persons at the camp are “ordered to leave the evacuation area immediately, and are further ordered not to return to the evacuation area”.

But Cecily Fong, spokeswoman for the North Dakota department of emergency services said, “We will not be using law enforcement or national guard to enforce the order.”

The 1985 law allows the governor to “direct and compel” evacuations “from any stricken or threatened area” in the event of natural or manmade disasters.

Dave Archambault, chairman of the Standing Rock Sioux tribal council, called the executive order “a menacing action meant to cause fear” and “a blatant attempt by the state and local officials to usurp and circumvent federal authority”.

“This is a clear stretch of state emergency management authority and a further attempt to abuse and humiliate the water protectors,” he said in a statement.

Thousands of Native American and environmental activists have gathered at the encampment, which is on Army Corps land just north of the Standing Rock Sioux Reservation and close to the site where the Dakota Access pipeline is slated to cross under the Missouri river.

On Friday, the Army Corps wrote to the Standing Rock Sioux tribe, ordering the closure of camps on its land, north of the Cannonball river. That announcement raised fears that law enforcement officials would attempt to forcibly evict the camps, but on Sunday, the Corps clarified that it had “no plans for forcible removal”.

As part of his order, the governor said, the state will not guarantee the provision of emergency services in the “evacuation area”.

“The general public is hereby notified that emergency services probably will not be available under current winter conditions,” he wrote.

A camp near the Standing Rock reservation. Photograph: Stephanie Keith/Reuters

Michael Knudsen, logistics coordinator for the Standing Rock Medic & Healer Council, which has provided care and services at the camps, said the announcement showed that the governor “has absolutely no empathy or care for the lives of the campers”.

“This is completely morally reprehensible,” he said, adding that the council was prepared to continue addressing medical needs at the camps without the help of the North Dakota government.

“We have never counted on the state’s help, because we know that ultimately the state health department is controlled by the power of the governor who has proven time and time again he does not want this encampment and therefore does not care about the wellbeing of the people.”

LaDonna Brave Bull Allard, a Standing Rock Sioux tribe member and founder of the Sacred Stone camp, said that the governor did not have authority over the land and that the water protectors would not evacuate.

“We are not leaving. We are standing. Ask every grandma who is standing out there,” said Allard, who owns land where some of the activists have been camped for months.

Allard drew parallels to the long history of colonizers forcing indigenous people off of their lands. “This time we are standing. This is taken land. We have a right to be there.”

She also scoffed at claims that the government was prioritizing public safety and the health of people camped at Standing Rock.

“Winter is a normal thing for us. The people are prepared and ready,” she said, noting that police recently deployed water cannons against nonviolent demonstrators. “They poured water over people when it was freezing.”

Allard said the announcement was an obvious ploy to help support the pipeline corporation, which has said that delays in construction have cost hundreds of millions of dollars.

“They are running out of time.”