At least 21 Chinese military aircraft entered Taiwan’s Air Defense Identification Zone (ADIZ) on the day US House Speaker Nancy Pelosi was scheduled to arrive on the island nation, according to Taiwan’s defence ministry.

Tensions between China and Taiwan intensified Tuesday after reports surfaced about Pelosi’s visit. China repeatedly warned the US of severe countermeasures if Pelosi visits Taiwan.

Also Read | China to conduct missile, military drills around Taiwan after Nancy Pelosi’s visit

Pelosi, who flew to Singapore on Monday as part of her tour of Asia, is due to spend Tuesday night in Taiwan. China had warned the US saying that the Chinese government will hold Washington accountable for any developments which would hurt Beijing’s sovereignty.

“What I can tell you is that the US will be held liable and pay the price for hurting China’s sovereignty and security interests,” Chinese foreign ministry spokesperson Hua Chunying said.

The ministry also issued a statement calling Pelosi’s visit to Taiwan “a serious violation of the one-China principle.”

“It has a severe impact on the political foundation of China-U.S. relations, and seriously infringes upon China’s sovereignty and territorial integrity,” according to the statement. “It gravely undermines peace and stability across the Taiwan Strait, and sends a seriously wrong signal to the separatist forces for ‘Taiwan independence’.”

Also Read | Russia condemns Nancy Pelosi’s expected Taiwan visit

Earlier on Tuesday, Taiwanese media CNA reported that the island nation ramped up its combat preparedness to take on the People’s Liberation Army (PLA) amid Pelosi’s impending visit. Meanwhile, the US also increased its military presence in the region.

In a statement Tuesday, Russia’s foreign ministry said China has the right to take measures to protect its sovereignty. Moscow had earlier labelled Pelosi’s trip a “provocation” and warned that it would put the US on a collision course with China.

Meanwhile, the People’s Liberation Army’s Eastern Theatre Command said it will conduct joint military operations near Taiwan from Tuesday night

“This action is targeted at the US’s shocking recent major escalation on the Taiwan issue, and serves as a serious warning to Taiwanese independence forces or those seeking independence,” Shi Yi, a spokesperson for the Eastern Theatre Command, said.