TAOYUAN, TAIWAN -- Taiwan welcomed 2.5 million COVID-19 vaccine doses from the United States on Sunday as help from a true friend, more than doubling the major semiconductor-producing island's arsenal of shots as it deals with a cluster of domestic infections.

Washington, competing with Beijing to deepen geopolitical clout through "vaccine diplomacy," initially had promised to donate 750,000 doses but increased that number as President Joe Biden's administration advances its pledge to send 80 million U.S.-made shots around the world.

A China Airlines Boeing 777 freighter carrying the vaccines landed at Taoyuan airport outside of Taipei late afternoon, greeted on the tarmac by Health Minister Chen Shih-chung and the top U.S. diplomat in Taiwan, Brent Christensen.

Speaking from her office in comments broadcast after the arrival of the Moderna Inc shots, President Tsai Ing-wen expressed her "sincere gratitude."

"A friend in need is a friend indeed," she added, switching into English.

The American Institute in Taiwan - the de facto U.S. embassy - said the donation reflected the U.S. commitment to Taiwan "as a trusted friend and an important security partner."

"As President Biden has said, 'we will not use our vaccines to secure favors from other countries,'" it said in a statement. "Instead, 'we'll share these vaccines in service of ending the pandemic everywhere.'"

Taiwanese politicians from across the political spectrum also thanked the United States, including the main opposition party, the Kuomintang, which has hammered Tsai for what it says is slow procurement of vaccines.

The United States, like most countries, has no formal diplomatic ties with Taiwan, but is its most important international backer, to China's anger.

The newly arrived 2.5 million doses will more than double the number of vaccines that have come to the island, including 1.24 million AstraZeneca Plc shots donated by Japan this month.

China, which claims democratically governed Taiwan as an integral part of its territory, has offered Chinese-made vaccines, but the government in Taipei has repeatedly expressed concerns about their safety and efficacy.

Despite the pandemic, China has continued to pressure Taiwan militarily, seeking to assert its sovereignty.

Last week, 28 Chinese air force aircraft, including fighters and nuclear-capable bombers, entered Taiwan's air defense identification zone, the largest reported incursion to date.

Taiwan is trying to speed up the arrival of the millions of vaccines it has on order as it deals with a rise in domestic cases, although infections remain comparatively low and have stabilized.

Only around 6 per cent of Taiwan's 23.5 million people have received at least one of the two-shot coronavirus vaccine regimen. (Reporting by Ann Wang; Writing and additional reporting by Ben Blanchard; Editing by William Mallard)