SHANGHAI -- US fast-food giants McDonald's and KFC said Thursday they are working with Chinese authorities to resolve allegations that the companies underpay their part-time workers, as a labor probe expands to other cities.

Labor bureaus in southern China's Guangdong province began investigating after the state-controlled newspaper New Express Daily reported that McDonald's, KFC and Pizza Hut were paying part-time workers less than the local minimum wage of about US$1 an hour.

The companies said they were seeking clarification of labor laws, while China's government-affiliated trade union demanded redress, noting that the probe had expanded to other provinces and cities, including Shanghai.

The ACFTU, the national umbrella group for government-approved unions, is in the midst of a campaign to boost the group's presence in foreign companies, which employ some 25 million people in China but until recently resisted allowing labor organizing.

McDonald's and Yum Brands Inc., which operates KFC and Pizza Hut, said they were in compliance with local laws, but were seeking clarification regarding recent changes in regulations.

"No one cares more about our workers than we do," McDonald's China said in a statement seen Thursday. "The fact is, government officials make decisions on local labor laws. That's why we continue to work with local officials to get further clarity on their recently amended labor laws," it said.

"McDonald's China always adheres strictly to the relevant national and local regulations, and we are consulting with the proper authorities to get the facts," it said, noting that the results of the investigation had not yet been disclosed.

A spokeswoman for Yum in China said the company was also seeking clarification but did not have any new statements.

Minimum wages vary from region to region in China. The minimum wage for Guangdong province is 7.5 yuan (US$0.97; euro0.73) per hour.

The New Express Daily report said McDonald's was paying part-timers, many of whom are college students, only 4 yuan per hour. It said part-timers at KFC earned as 4.7 yuan and Pizza Hut 5 yuan.

The report also accused the restaurants of demanding part-time employees work the hours of full-time staff but failing to pay them any full-time staff benefits.