The closure of the plant and other slaughterhouses due to coronavirus cases among employees has led to temporary shortages of US meat products and left farmers without markets for their hogs, forcing some to euthanise livestock.

Smithfield, owned by China's WH Group Ltd, reopened the ground seasoned pork and night cleanup departments at the plant in Sioux Falls, about three weeks after saying the facility would shut indefinitely, a spokeswoman for the United Food and Commercial Workers said.

About 250 of the plant's 3,700 workers went back to work, she said. The company plans to gradually restart more operations and has not resumed slaughtering hogs, according to the union.

The company said in an emailed statement it had not resumed operations at the facility, which accounted for about 5 percent of US pork production.

Smithfield made coronavirus testing optional for employees to go back to work, South Dakota Health Secretary Kim Malsam-Rysdony said on the webcast of a news conference. State and union officials are urging employees to be tested.

South Dakota will hold a mass testing event for Smithfield employees and their dependants, South Dakota Governor Kristi Noem said on the webcast.

"One of the most effective things that we can do is test Smithfield employees to make sure they are COVID negative before they're going back into work," Noem said. "We do encourage every single one of them to participate."

The US Centers for Disease Control and Prevention did a walk-through of the plant on Monday morning, Noem said.

The CDC has said 19 states reported COVID-19 cases in 115 meat and poultry processing facilities. Among about 130,000 workers at the facilities, about 5,000 cases and 20 deaths occurred. COVID-19 is the respiratory disease caused by the novel coronavirus.

"As testing becomes more widely available, consideration should be given to its role in rapidly identifying and addressing COVID-19 in this occupational setting," the CDC said.