BEIJING (Reuters) - Religious extremism has begun to spread to inland China from its western Xinjiang region, long considered by the government to be at the forefront of its efforts to battle Islamist separatists, the country's top religious affairs official said.

China says it faces a serious threat from Islamist militants in Xinjiang, which borders central Asia, Afghanistan and Pakistan, and is home to the largely Muslim Uighur minority group.

Hundreds have died there in recent years in violence that Beijing blames on religious extremists, and the government has put in place tight controls on religion in the name of combating radicalism and maintaining stability.

Extremist thought was now infiltrating China's "inland provincial areas", Wang Zuoan, head of the State Administration for Religious Affairs, told the National Congress of the Chinese Islamic Association, according to an article in the official China Daily newspaper on Monday.

The paper did not give details of the spread or mention specific provinces, but cited Wang as saying China's official Islamic clergy must be the "front line" in fighting extremism and should work to "convert" those influenced by it.

"We should let Muslims know the boundaries between legal and illegal religious activities, to enable them to say no to illegal activities," he told the association on Saturday.

Wang also said that China must "appropriately manage the issue of Menhuan Sect Islam", referring to a Chinese-style Sufism, a mystical form of Islam, according to a statement posted on the State Administration of Religious Affairs website.

China has about 21 million Muslims, only a portion of which are Uighur. Other Muslim groups, such as the Hui, are spread throughout the country, including in the western region of Ningxia and the southwestern province of Yunnan.

President Xi Jinping has urged Chinese Muslims to resist illegal religious "infiltration".

Attacks tied to such extremism by the government have spread beyond Xinjiang in recent years. A grisly knife attack in Yunnan's Kunming train station in March 2014 killed more than 30 people.

Rights groups say that unrest in Xinjiang often stems from localized incidents that boil over into physical violence, fueled by ethnic tension and religious and economic repression of Turkic-language speaking Uighurs.

Beijing regularly denies religious discrimination against minorities in Xinjiang or elsewhere. Despite a resurgence of religious faith, officially atheist China restricts believers to a handful of recognised religions overseen by the state.

(Reporting by Christian Shepherd and Michael Martina; Editing by Nick Macfie)