Strikes by thousands of teachers over salary and pension disputes have spread to half a dozen cities and counties in a Chinese province, state media reported Monday.

The protesters marched on a government building in Yilan county in Heilongjiang at the weekend, the Global Times reported, while primary and high school teachers in five other counties and cities in the northeastern province also went on strike.

Some schools would be closed on Monday, added the paper, which is close to the ruling party.

Photos circulated on social media showing strikers holding a sign saying, "4,000 for Teachers' Rights, Return Our Withheld Money".

China's Communist Party is wary of an independent labour movement so only allows one government-linked trade union, which in the past has acted to prevent workers from striking.

But analysts say that in recent years workers have become more empowered as labour shortages give them bargaining power -- although strikers still risk police detention.

Schools threatened teachers with dismissal and demotions to try to stop them taking part in the protests, the Global Times said.

The teachers are demanding an increase in salaries and the return of money they were obliged to pay into a pension scheme under a pilot scheme in the province, according to a report by the official China National Radio.

The strikes come just weeks after another work stoppage in nearby Zhaodong city forced authorities to increase teachers' monthly pay by an average 772 yuan ($125).