Two Chinese balloons flew north of Taiwan, the island’s Defense Ministry said Monday, as China increases pressure on the self-ruled territory it claims as its own ahead of a presidential election in January. The balloons crossed the sensitive Taiwan Strait separating the island from China and were detected about 110 nautical miles (204 kilometres) northwest of the northern port city of Keelung on Sunday, the ministry said.

It was the second time this month Taiwan reported a Chinese balloon near its territory, after one crossed southwest of Keelung on December 7. China’s potential for using weather balloons to spy on other governments came into focus earlier this year, when the United States shot down a suspected Chinese spy balloon over the Atlantic Ocean, drawing China’s ire.

Taiwan’s Defense Ministry did not say whether it suspected the balloon could have been used for spying. China’s Foreign Ministry did not immediately comment. Beijing has increased pressure on Taiwan by sending military aircraft and warships near the island almost daily.

Taipei is preparing for a presidential election on January 13. The front-runner, current Vice President William Lai of the independence-leaning Democratic Progressive Party, has been criticised by Beijing as a “separatist.” Lai has said he wants to maintain peace and the status quo in relations with China.

Taiwan split from China amid civil war in 1949, but Beijing continues to regard the island of 23 million with its high-tech economy as Chinese territory and has been steadily upping its threat to achieve that goal by military force if necessary.