The comments are the first from Washington on relations with Taiwan since President Joe Biden's inauguration last week, and came on the same day Taipei reported multiple Chinese jets and bombers had flown into its air defense zone.

State Department spokesman Ned Price said it "notes with concern the pattern of ongoing PRC attempts to intimidate its neighbors, including Taiwan" and "urges Beijing to cease its military, diplomatic, and economic pressure against Taiwan."

The statement added that Washington would continue to work on "deepening" ties with democratic Taiwan.

Taiwan split from China at the end of a civil war in 1949. Its 23 million people live under the constant threat of invasion by the mainland, whose leaders view the island as their territory and have vowed to one day take it.

Washington diplomatically recognizes Beijing over Taipei, but remains the latter's most important unofficial ally and military backer.

Beijing baulks at any official contacts with Taiwan and tries to keep the island diplomatically isolated.

President Donald Trump embraced warmer ties with the island as he feuded with China on issues like trade and national security.

Beijing has called for a reset in US-China ties under Biden and on Thursday warned the US to "cautiously and appropriately handle Taiwan issues to prevent harm to US-China relations".

But the new US president is expected to remain tough on the superpower rival despite a softening of diplomatic tone.

Protecting Taiwan has become a rare bipartisan issue, especially as Chinese President Xi Jinping has ramped up threats towards the island.

Last year, Chinese airplanes made a record 380 incursions into Taiwan's air defense identification zone and there is little sign of a shift since Biden came to power.

Taiwan's Ministry of National Defense said 13 People's Liberation Army aircraft, including eight bombers and four jets, entered its defense zone on Saturday.

The New York Times said China also sent 15 planes -- including 12 fighters -- into the zone on Sunday.

In a tweet on Sunday, Taipei's Ministry of Foreign Affairs thanked the US for its statement of support "in the face of Beijing's ongoing coercion."

There was no immediate response from Beijing on Sunday.

In a subtle but symbolic gesture, Taiwan's de facto ambassador to the US was formally invited to Biden's inauguration in what Taipei said was a precedent-setting first since Washington switched recognition to Beijing in 1979.

The US statement on Saturday noted that Washington would "continue to assist Taiwan in maintaining a sufficient self-defense capability".