China arrested more than 1,000 people last year for "endangering state security", the country's most serious political crime, an advocacy group reported this week.

In 2012, Chinese authorities arrested 1,105 people in 474 cases related to the crime – a 19% rise over 2011, the San Francisco-based organisation Dui Hua has reported, citing the official China Law Yearbook.

The increase apparently relates to rising tensions on the country's ethnically divided borderlands. Last year, three-quarters of the country's trials for endangering state security – often called ESS – were held in Xinjiang, a restive region in north-west China that's home to the predominantly Muslim Uighur minority.

The arrests are intended to "resolutely fight the crimes of splittism, subversion, terrorism and all kinds of cult organisations in accordance with the law to maintain state security and social and political stability, consolidate the party's ruling position, and defend the socialist regime," the China Law Yearbook said, according to Dui Hua.

China introduced ESS to its law books in 1993 to replace a raft of outdated "counter-revolutionary" offences. The charge combines 12 individual crimes, including subversion, splittism and treason.

Although China classifies information on ESS cases as a state secret, Dui Hua described 17 people arrested for the crime last year in its Political Prisoner Database, a record of China's political and religious prisoners since 1980.

The organisation listed five Han Chinese, five Uighur, five Tibetan and two ethnic Mongolian detainees, and their charges seemed to fall along ethnic lines. "Due to real and perceived independence movements, Uygurs and ethnic Tibetans bear the brunt of crackdowns on splittism," said Dui Hua.

The Tibetan arrests seemed to align with a spate of self-immolation protests that have gripped the Himalayan region in recent years. One of the ethnic Mongolians is Wang Lijun, a top leader in the south-western metropolis Chongqing, who defected to a US consulate, precipitating the downfall of his superior, Bo Xilai; the other, a political activist, Govruud Huuchinhuu, was charged with leaking state secrets.

Joshua Rosenzweig, an independent rights scholar based in Hong Kong, said he found the spike in arrests unsurprising given widespread awareness of rising tensions in Xinjiang and Tibet. "But it's one thing to have anecdotal evidence of individual incidents, and it's another thing to have statistical evidence that confirms that the situation may be even more serious than those few reports suggest," he said.

While the proportion of trials in Xinjiang has dropped from 86% in 2011, Dui Hua said that large groups of defendants can be tried simultaneously, meaning the total number of arrests in the region has not necessarily decreased.

Xinjiang experienced 190 terrorist attacks in 2012, a significant increase from 2011, according to Oriental Outlook, a magazine published by China's official newswire Xinhua.