(CNN) China pressed ahead with military drills near Taiwan and flew drones close to Japan on Friday, escalating tensions in the strait that have threatened to boil over since Taipei welcomed US House Speaker Nancy Pelosi on a historic visit.
Taiwan's defense ministry said Chinese warships and aircraft continued drills in waters around the island, adding that Chinese People's Liberation Army (PLA) forces crossed the median line -- the halfway point between the island and mainland China -- in a move it called a "highly provocative act."
The line has previously been an informal but largely respected border of control between Beijing and Taipei. Taiwan's military responded with radio warnings and put air patrol forces, naval ships and shore-based missile systems on alert, the ministry added.
Two Chinese drones also flew around Okinawa prefecture in Japan on Thursday, prompting the country's Air Self-Defense Force to scramble fighter jets in response, according to a statement from Tokyo's Ministry of Defense on Friday.
In all, 49 planes entered its air defense identification zone (ADIZ), Taiwan's Ministry of National Defense said in a statement. The ministry had previously suggested 68 aircraft flew into the area, but later clarified that 19 operated west of the median line dividing the Taiwan Strait.
The moves comes as China doubles down on rhetoric blaming the US for spiking tensions and forcing it into a response. Beijing announced sanctions against Pelosi and her immediate family and said it would suspend cooperation with Washington on several issues, including fighting climate change, on Friday.
Taiwan's Premier Su Tseng-chang said the island represented "freedom and democracy," and that "the evil neighbor next door flexed its muscles on our doorstep and arbitrarily sabotaged (one of) the busiest waterways in the world with military exercises."
The skies and waters around Taiwan have become a focal point as Beijing ramps up tensions not only with Taiwan, but with neighboring Japan, which lodged a formal protest with China after five projectiles landed in Japan's Exclusive Economic Zone (EEZ).
The missiles were among a number launched by Beijing Thursday -- some of which flew over Taiwan -- as Pelosi made her way to Tokyo where she met with Japanese Prime Minister Fumio Kishida on Friday.
China has previously fired missiles into waters surrounding Taiwan -- a democratic island of 24 million that the Chinese Communist Party regards as its territory, despite having never controlled it -- most notably during the Taiwan Strait Crisis in the 1990s.
But missiles flying over the island marked a significant escalation, with US officials warning there may be more to come.
"We anticipated that China might take steps like this -- in fact, I described them for you in quite some detail just the other day," John Kirby, a spokesperson for the US National Security Council, told reporters at the White House on Thursday. "We also expect that these actions will continue and that the Chinese will continue to react in the coming days."
A US aircraft carrier will stay in the area around Taiwan for several more days to "monitor the situation," Kirby added.
On Friday, Kishida said the Chinese military drills were "a serious issue concerning the security of our country and its people" and called for an immediate halt to the exercises. Japan and the US would "work together to maintain stability in the Taiwan Strait," he added.
Speaking in Tokyo on Friday, Pelosi accused China of trying "to isolate Taiwan," pointing to the island's exclusion from international groups like the World Health Organization.
"They may try to keep Taiwan from visiting or participating in other places, but they will not isolate Taiwan by preventing us to travel there," she said.
She added that her visit to Taiwan had been about maintaining the status quo, not changing it.
Later on Friday, China's Foreign Ministry said it was imposing sanctions on Pelosi and her immediate family for "seriously interfering in China's internal affairs (and) seriously undermining China's sovereignty and territorial integrity."
China started military drills around the island on Thursday, firing multiple missiles toward waters near northeastern and southwestern Taiwan the day after Pelosi's departure.
A Chinese military expert confirmed on state broadcaster CCTV that the conventional missiles flew over Taiwan's main island, including airspace covered by Taiwanese defense missiles.
"We hit the targets under the observation of US Aegis combat system, which means the Chinese military has solved the difficulties of hitting long-range targets on waters," said Maj. Gen. Meng Xiangqing, a professor of strategy at the National Defense University in Beijing.
In a statement late Thursday, Taiwan's Defense Ministry said the missiles had traveled above the atmosphere and therefore posed no risk to the island.
Authorities did not trigger air raid alerts because they predicted the missiles would land in waters east of Taiwan, the ministry said. The ministry added it would not release further information about the missiles' trajectory to protect its intelligence-gathering capabilities.
Five ballistic missiles are believed to have landed within Japan's EEZ, including four believed to have flown over Taiwan, said Japan's Defense Ministry on Thursday.
"This is a serious problem that concerns Japan's security and the safety of its citizens. We strongly condemn it," Japan's Defense Minister Nobuo Kishi told reporters during a news conference.
China also sent 22 warplanes into Taiwan's air defense identification zone (ADIZ) on Thursday -- all of which crossed the median line.
It follows similar Chinese incursions a day earlier across the median line.
Thursday's incursions were made by 12 Su-30 fighter jets, eight J-11 fighter jets, and two J-16 fighter jets, Taiwan's Defense Ministry said in a statement.
Later Thursday, the ministry said it detected four drones flying over "restricted waters" around the Taiwan-controlled Kinmen islands near mainland China. The ministry said Taiwan's military fired flares to warn the drones away, but did not specify the type or origin of the devices.
On Friday, a spokesperson from Taiwan's Foreign Ministry said the government's English website and the ministry website had both detected a large number of attacks on Tuesday -- the day Pelosi landed in Taiwan. The IP addresses behind the attacks came from China and Russia, among other locations, with the intention of paralyzing the government sites, said the spokesperson.
She added that the websites were back to normal after emergency repair work, but "massive attacks with malicious intent .. by overseas hostile forces" continued on Thursday and Friday.
In a speech on Thursday, Taiwan's President Tsai Ing-wen condemned China's military drills as "irresponsible," saying they marked a "deliberate and continuous escalation of military threats."
"I must emphasize that, we do not seek to escalate conflicts or provoke disputes, but we will firmly defend our sovereignty and national safety, as well as safeguarding democracy and freedom," she added.
She also thanked the Group of Seven, comprised of the world's largest economies, which released a statement on Wednesday voicing concern over China's live-fire exercises and urging Beijing not to change the status quo in the region.
The exercises have also caused disruption to flight and ship schedules, with some international flights canceled and vessels urged to use alternative routes for several ports around the island.
On Tuesday, the Chinese Defense Ministry said it would hold its exercises in six zones around Taiwan, warning ships and aircraft to stay out of the areas during the drills.
The Taiwan Strait is a key trade route for vessels carrying goods between major economies in northeast Asia such as China, Japan, and South Korea, and the rest of world.
This story has been updated with new information.