U.S. pork prices rose in recent weeks at a time when they would normally be falling, as a fatal pig disease in China is tightening global meat supplies, the chief executive of Tyson Foods Inc. said on Tuesday.

The price increase is probably the first time African swine fever (ASF) has significantly affected the United States, CEO Noel White said, after the company posted lower-than-expected quarterly earnings.

The increase is "extremely unusual," White told analysts on a conference call, because prices usually drop in the autumn. The United States also has more hogs than ever. Processors are slaughtering about 2.7 million pigs per week, up from about 2.6 million a year ago.