BEIJING (REUTERS) - Large numbers of pigs are dying from the African swine fever in China's top hog-producing province, say farmers and analysts, raising concerns it could spread further across the south and slow China's pork production recovery.

The deadly African swine fever virus wiped out around half of China's huge pig herd during 2018 and 2019, but the country rapidly rebuilt much of the lost stock last year.

But there have been fresh outbreaks in northern China this year, and there are more strains of the virus circulating.

Now, south-western Sichuan province, which produced 48.5 million hogs for slaughter last year, about 9 per cent of the country's total, is also seeing a resurgence of the virus.

"Recently Sichuan is quite serious," said Ms Xiao Lin, analyst at Shenzhen-based Win & Fun investment fund.

Most of those impacted are small farmers who had relaxed disease prevention measures to reduce costs after hog prices plunged in recent months, she said.

Small farmers still account for a relatively large share of hog output in Sichuan, where mountainous terrain limits the number of large farms.

Ms Xiao estimated losses at around 10 per cent to 15 per cent of the herd.

"It started in June and is still spreading," said a farmer in Jingyan county under Leshan city who had more than 30 sows and several hundred hogs before the disease hit his farm.

"The big pigs all got it and died. There's still more than a hundred small ones that might also die," he told Reuters, declining to be named because of the sensitivity of the issue.

He said the disease, which he had already experienced in 2019, had infected almost all farms nearby, and many several kilometres further away.

"There are dead pigs everywhere," said another farmer in Luzhou, a city 200 km south-east of Jingyan.

The Sichuan Provincial Department of Agriculture and Rural Affairs did not respond to a request for comment.

China's Ministry of Agriculture and Rural Affairs reported two cases of swine fever in Sichuan in March, one in the east in Huaying city, and another in the far west.

It did not respond to a fax seeking comment on recent cases.

Sichuan's government said this week it was introducing measures to stabilise hog production and pork prices, including "deepening prevention and control of African swine fever and other major diseases". It is targeting output of 58 million pigs this year.

The latest outbreaks have created additional supply as farmers panic and send pigs to slaughter. Sichuan's hog price, usually higher than most regions because of its large population and high consumption levels, has fallen below the national average to 15.6 yuan (S$3.30) per kg this week.

Ms Xiao said she is watching closely in case swine fever outbreaks spread to nearby Guangxi and Guangdong, both significant hog-producing regions.

"This won't stop the recovery of the pig herd but it could impact the pace," she said.