China was scheduled to hold live-fire drills in the Taiwan Strait Wednesday, flexing its military muscle after warning Taipei about seeking independence or closer ties with Washington.
It is the first such exercise in the waterway since 2016, and it coincides with Taiwanese President Tsai Ing-wen's visit to Swaziland, one of the self-ruled island's few remaining international allies.
The Chinese government has given scant details about the manoeuvres, with a Fujian province maritime safety administration statement merely saying last week that the day-long drills would start at 8:00 am (0000 GMT).
China's Taiwan Affairs Office director Liu Jieyi said Monday the drill was "an action to safeguard the sovereignty and territorial integrity of our motherland".
As she left for Africa on Tuesday, Tsai said Taiwan has "the confidence and determination to safeguard the country's security".
Relations between Beijing and Taipei have deteriorated since Tsai came to office in May 2016, largely because she has not embraced the position that Taiwan and China are part of one country.
China sees the island as a renegade part of its territory to be brought back into the fold and has not ruled out reunification by force.
"It is likely that these drills were planned months ago, but they are a useful warning to Taiwan and the US to not cross Chinese red lines," China expert at the Washington-based Center for Strategic and International Studies, told AFP.
Tsai's Democratic Progressive Party (DPP) is traditionally pro-independence and her newly appointed premier William Lai is a long-standing independence advocate.
After the Communist Party-led parliament paved the way for Xi Jinping to rule for life, the Chinese president warned on March 20 that "all acts and tricks to separate the country are doomed to fail".
That same day, China's sole operational aircraft carrier, the Liaoning, sailed through the Taiwan Strait.
"The mainland must create military pressure to let the other side know that no matter whether it happens gradually or they really declare independence, it is totally unacceptable," Song Zhongping, military commentator for Hong Kong-based Phoenix TV, told AFP.
Song, a former lecturer at a People's Liberation Army university, said the Liaoning is likely to participate in Wednesday's drill, as it "has a lot of advantages for resolving the Taiwan problem."
"It can effectively acquire control of the airspace, and even effectively block the US-Japanese alliance's strategy for intervening in China's plan to settle the Taiwan issue," he said.
A flotilla including the Liaoning has conducted combat training in the South China Sea in the past few days, the navy said on Tuesday.
Beijing has stepped up military patrols around Taiwan and used diplomatic pressure to isolate Taiwan internationally since Tsai took office, with Panama switching allegiance to China last year.
Swaziland is among just 20 nations that still recognise Taipei.
The drill "is part of Beijing's psychological warfare against Taiwan, and possibly a means to divert attention from Tsai's visit abroad by compelling media to report on the military drills," said J. Michael Cole, a Taipei-based senior fellow at the University of Nottingham's China Policy Institute.
The planned drill could also serve as a signal to Washington, which sent an aircraft carrier through the disputed South China Sea last week.
Washington switched diplomatic recognition from Taiwan to China in 1979 but maintains trade relations with the island and is its main arms supplier.
Beijing protested last month after President Donald Trump signed a bill allowing top-level US officials to travel to Taiwan.
China said the US should stop official exchanges with Taiwan to avoid "damaging Sino-US relations".