The democratic self-governed island has complained of repeated military activities by Beijing in recent months, with China's air force making almost daily sorties into Taiwan's air defence identification zone.

On Monday, China said an aircraft carrier group was exercising close to the island.

Taiwan's Defence Ministry said 15 Chinese aircraft including 12 fighters entered its air defence identification zone, with an anti-submarine aircraft flying to the south through the Bashi Channel between Taiwan and the Philippines.

Taiwan's air force sent up aircraft to intercept and warn the Chinese away, the ministry added.

Speaking earlier in the day, Mr Wu said the United States was concerned about the risk of conflict.

"From my limited understanding of American decision-makers watching developments in this region, they clearly see the danger of the possibility of China launching an attack against Taiwan," he told reporters.

"We are willing to defend ourselves without any questions and we will fight the war if we need to fight the war. And if we need to defend ourselves to the very last day we will defend ourselves to the very last day."

China's Taiwan Affairs Office and the US State Department did not respond to requests for comment on Mr Wu's remarks.

China says its activities around Taiwan are aimed at protecting China's sovereignty. The US has expressed concern about China's actions, saying its commitment to Taiwan is "rock solid".

China does not recognise Taiwan's democratically elected government, and leader Xi Jinping has said "unification" between the sides cannot be put off indefinitely.

"On the one hand, they want to charm the Taiwanese people by sending their condolences, but at the same time they are also sending their military aircraft and military vessels closer to Taiwan aimed at intimidating Taiwan's people," Mr Wu said at a ministry briefing.

"The Chinese are sending very mixed signals to the Taiwanese people and I would characterise that as self-defeating."

'If we need to open fire, we open fire'

Lee Chung-wei, who heads the Ocean Affairs Council under whose purview the Coast Guard falls, told Parliament that Chinese drones had been spotted circling the Taipei-controlled Pratas Island in the South China Sea.

"They have never entered our restricted waters and airspace, they've just flown around them at a certain distance," Vice Admiral Lee said.

While China recognises no Taiwanese claims of sovereignty, its aircraft and ships generally stay outside Taiwan's restricted zone, which extends 6 kilometres from its coast.

Asked how the Coast Guard would react if a Chinese drone entered that restricted zone, Vice Admiral Lee said they had rules of engagement.

"After it enters it will be handled under the rules. If we need to open fire, we open fire."

The Pratas lie at the top end of the disputed waterway and have become a relatively new source of intrigue between Chinese-claimed Taiwan and Beijing.

In recent months, Taiwan has complained of repeated Chinese air force activity near the islands, which Taiwan's Coast Guard only lightly defends, though there are periodic deployments of marines. There is no permanent civilian population, only occasional visiting scientists.

In October, Hong Kong air traffic controllers warned off a Taiwanese civilian flight flying to the Pratas on a routine weekly supply run, forcing it to turn back.

The Pratas, the closest Taiwan-controlled territory to Hong Kong, have also taken on extra significance since anti-government protests began in the Chinese-run city.

Taiwan has intercepted at least one boat close to the Pratas carrying people fleeing from Hong Kong trying to make their way to Taiwan.

Taiwan's other main South China Sea island is Itu Aba, also known as Taiping Island, which is part of the Spratly archipelago.

Vice Admiral Lee said Chinese drones had not been spotted there.

US issues warning over China's intentions

The vast improvement in China's military capabilities, and its increasing activity around Taiwan, have raised concerns in the US, which is legally bound to ensure Taiwan is capable of defending itself and to regard all threats to the island's security as matters of "grave concern".

The American military warned on Wednesday that China was probably accelerating its timetable for capturing control of Taiwan, which has been the chief source of tension between Washington and Beijing for decades and is widely seen as the most likely trigger for a potentially catastrophic US-China war.

On Monday, China's military said the new naval drills were meant to help it "safeguard national sovereignty, security and development interests," terms often interpreted as being directed at Taiwan's leadership, which has refused to give in to Beijing's demands that it recognise the island as part of Chinese territory.

Taiwan and China split amid civil war in 1949 and most Taiwanese favour maintaining the current state of de facto independence while engaging in economic exchanges with the mainland.

China has created conditions for greater economic integration while also targeting some communities, such as pineapple farmers, in hopes of weakening their support for the island's government.

Chinese diplomatic pressure has been growing also, reducing the number of Taiwan's formal diplomatic allies to just 15 and shutting its representatives out of the World Health Assembly and other major international forums.

Taiwan has responded by boosting its high-tech industries and unofficial foreign relations, particularly with its key partners the US, Japan and others, and by building up its own defence industries, including a submarine development program, while buying upgraded warplanes, missiles and other military hardware from the US.

Predictions of when China might decide to try to compel Taiwan to reunite with the mainland have long varied, and there is no uniform view in the United States.

Larry Diamond, a senior fellow at Stanford University's Hoover Institution, said last week he doubted Chinese leaders were ready to force the issue.

"I don't think it's coming soon," he said.