China's education minister has issued a stern warning against threats to communist ideological purity in higher education, saying Western values must never be permitted to infiltrate the classroom.

Yuan Guiren reiterated the need for strict ruling Communist Party control over education and said educators must ensure that teaching materials do not criticize the party's leadership or the socialist system, the official Xinhua News Agency said in a report late Thursday.

Professors must "not complain, vent personal grievances or convey negative emotions to their students," Xinhua quoted Yuan as saying. It said he demanded teachers maintain a firm bottom lines in politics, law and morality, but offered no specifics.

Yuan's remarks at an academic conference on Thursday come amid tightening political restraints under party leader Xi Jinping, who has moved quickly to enforce communist orthodoxy since taking charge in 2013, despite maintaining the country's economic openness.

An internal party document leaked in 2013 warned against Western values such as constitutionalism, respect for civil society and press freedom, and state media outlets have lashed out at professors who take critical stances.

Academics who fail to toe the line have seen their careers and even personal freedoms come under threat. At least three Beijing-based professors have been fired or otherwise disciplined for their teachings about sensitive topics, such as the Arab Spring uprisings and constitutionalism in China.

Economics professor Ilham Tohti was sentenced to life in prison in September on separatism charges in part for championing the rights of the country's Muslim Uighur minority during his lectures at Minzu University in Beijing. Seven of his students, all Uighurs, were sentenced to three to eight years on separatism charges.