TAIPEI (Reuters) - China has sailed a carrier group into the sensitive Taiwan Strait led by its first domestically build aircraft carrier as election campaigning kicked into high gear on the self-ruled island on Sunday.

FILE PHOTO: Taiwan's President Tsai Ing-wen visits an exhibition of Taiwanese products in Port-au-Prince, Haiti July 13, 2019. REUTERS/Andres Martinez Casares/File Photo

Taiwan’s foreign minister, Joseph Wu, said they would not be intimidated.

Democratic Taiwan is claimed by China as a wayward province and is the Communist Party’s most sensitive and important territorial issue. China has threatened to attack if Taiwan moves toward independence.

Taiwan’s defense ministry announced the sailing in the strait just hours after President Tsai Ing-wen named as her running mate for 2020 elections a former premier who angered Beijing so badly last year with his support for Taiwan’s formal independence that a major Chinese paper called for his arrest.

The Chinese carrier group had sailed in a southerly direction through the Taiwan Strait, trailed by U.S. and Japanese ships, Taiwan’s defense ministry said in its short statement without giving details on exactly when it happened.

The island scrambled ships and aircraft to monitor the group and “ensure national security and safeguarding of regional peace and stability”, it added.

China’s Defence Ministry did not immediately respond to a request for comment.

The still-unnamed carrier was launched last year, but Chinese military experts have told state media it is not expected to enter service until 2020, once it has been fully kitted out and armed.

A Japan Self Defence Forces spokesman said he had no information about the movement of the Chinese carrier or any Japanese ships nearby.

Speaking earlier in the day in Bangkok, U.S. Defense Secretary Mark Esper slammed China’s behavior broadly during defense talks with his Japanese and South Korean counterparts.

“Beijing is increasingly resorting to coercion and intimidation to advance its strategic objectives, at the expense of other nations,” Esper said, without mentioning the Chinese carrier passage.

“NO TO CHINA”

Taiwan’s Wu said China was intending to intervene in their elections, just as Tsai had named her running mate and “the campaign shifts into high gear”.

“Voters won’t be intimidated! They’ll say NO to China at the ballot box,” he tweeted.

Tsai had begun the day by announcing her vice-president candidate, William Lai, premier until January when he stepped down to take responsibility for a defeat in regional elections last November for the ruling Democratic Progressive Party.

In April last year, while still premier, Lai told parliament he was a “Taiwan independence worker” and that his position was that Taiwan was a sovereign, independent country.

China’s influential Global Times tabloid responded by saying China should issue an international arrest warrant for him to face prosecution under the country’s 2005 Anti-Secession Law.

Accepting Tsai’s nomination, Lai made no mention of independence, but said the island had to stand up to pressure from an encroaching China and “show the way” for Chinese-ruled Hong Kong, which has seen months of anti-government protests.

“In the face of a China which is closing in at every stage and warning signs from Hong Kong, what can the Democratic Progressive Party in next year’s critical election battle give the people of Taiwan? Isn’t our most important mission to unite and defend Taiwan?” Lai said.

“I have decided to accept President Tsai’s invitation, to be her deputy, to team up to fight the election, and at this darkest time to unite and defend Taiwan, to continue to show the ray of light of democracy, show the way for Hong Kong and illuminate the world.”

Taiwan’s main opposition party, the China-friendly Kuomintang, said Tsai was seeking to gather “extremist” votes by trying to cosy up to “Taiwan independence forces” with her choice of Lai as running mate.

China has stepped up its pressure on Taiwan ahead of the elections. President Xi Jinping said in January that China reserves the right to use force to bring Taiwan under its control but will strive to achieve peaceful “reunification”.

While the Democratic Progressive Party is pro-independence, Tsai has said she wants to maintain the status quo with China, though will defend Taiwan’s security and democracy.