Taiwan must continuously increase its defense spending if it is to have the ability to deter and repel a cross-strait invasion, David Helvey, US principal deputy assistant secretary of defense for Asian and Pacific security affairs stressed Monday.

China has turned up the pressure on Taiwan, both diplomatically and militarily.

Chinese President Xi Jinping has urged troops in southern China to prepare for war while the Chinese defense minister has stressed that China will not cede a "single inch" of territory.

The US has already approved two arms sales packages to Taiwan in the past 18 months, as the US attempts to establish a "more normal foreign military sales relationship," as Randall Schriver, the US assistant secretary of defence for Asian and Pacific security affairs, said earlier this month.

Beijing has strongly criticized closer ties between the US and Taiwan.

A Pentagon official is urging Taiwan to ramp up its defence budget to protect against potential attacks from mainland China.

David Helvey, US principal deputy assistant secretary of defence for Asian and Pacific security affairs, told a conference in Annapolis, Maryland on Monday that the self-ruled island “must have resources to modernise its military and provide the critical materiel, manning and training needed to deter, or if necessary defeat, a cross-strait invasion”.

Helvey said “Taiwan’s current efforts will falter” unless its defence budgets keep growing, according to a transcript of his remarks released by the US-Taiwan Defence Industry Conference on Tuesday.

The official also accused China of attempting to “erode Taiwan’s diplomatic space in the international arena while increasing the frequency and scale of [People’s Liberation Army] activity within and beyond the First Island Chain.”

The first island chain refers to the first chain of major archipelagos, including Taiwan and Japan, out from the East Asian continental mainland coast.

Helvey said that by stepping up Beijing's ability to strike with advanced equipment, including long range bombers, the PLA was trying to end the era of zero military conflict that had existed in the Taiwan Strait since 1949, when the mainland’s Communist Party won a civil war that drove the Nationalist government to the island.

“These coercive acts only make more difficult the peaceful resolution of cross-Strait issues in a manner and scope, and at a pace, acceptable to the people on both sides,” the official said.

Under the Taiwan Relations Act, the US has been selling arms to Taiwan to maintain the island’s self-defence capability as part of an overall effort to prevent the mainland from taking it over by force.

Helvey’s comments come four days after President Xi Jinping, during an inspection tour in Guangdong province, told the Chinese military region responsible for monitoring Taiwan and the South China Sea to “prepare for war”.

Defence Minister Wei Fenghe also vowed on Thursday not to cede “a single inch” of territory to Taiwan, which the mainland has consistently regarded as a wayward province, to be brought into line by force if necessary.

Wei is expected to meet US defence chief James Mattis in Washington next week. Pentagon spokesman Christopher Logan said in an email that Mattis has invited Wei to visit the US capital during the week of November 5.

Helvey told the defence industry meeting that the US and Taiwan both needed to update their thinking on arms procurement, planning and training to counter a potential strike against the island by mainland forces.

“These changes are essential if we are to look dispassionately at the military balance in the region and devise a way ahead that ensures Taiwan has the ability to resist coercion and deter aggression,” the Pentagon official said.

US President Donald Trump has approved two separate packages of arms sales to the island in less than 18 months. The first, valued at US$1.4 billion, came in June 2017, the second, worth US$330 million, in September.

Rupert Hammond-Chambers, president of the US-Taiwan Business Council, speculated on Sunday during a press conference that the US government would likely notify the US Congress of new arms sales to Taiwan, worth at least US$50 million, sometime before the end of this year.

The Washington-area business council is a non-profit group advocating for closer business ties with the self-governing island, focusing on defence procurement. The US is Taiwan’s sole arms supplier even though no formal diplomatic relations exist between the two.

Taiwan’s deputy defence minister, Chang Guan-chung, who led a Taiwanese delegation to the two-day Annapolis conference, declined to confirm whether a new arms deal with the US would occur this year.

York W. Chen, the deputy secretary general with Taiwanese President Tsai Ing-wen's National Security Council, told reporters the Taiwanese government would welcome future new arms sales by the US.

He also said Taiwan would welcome “normalised and multilevel” arms sales with the US based on a case-by-case review of requests for weapons purchases. A new system could result in more “frequent” sales of arms to Taiwan, Chen said.

The Pentagon already has advocated for establishing such a system for its arms sales to Taiwan. Randall Schriver, the US assistant secretary of defence for Asian and Pacific security affairs, was quoted by Taiwan’s Central News Agency this month as saying the US was moving towards a “more normal foreign military sales relationship” with the island.

China strongly opposes any US arms sales and official contact with Taiwan. Chinese Foreign Ministry spokesman Lu Kang last week urged the US “to correct its mistakes, stop any official contact and military ties with and arms sales to the Taiwan region.”

Meanwhile, Taiwan has continuously expressed a desire to acquire advanced American weapons. Its defence ministry has said it needs M1A2 Abrams battle tanks and has suggested it plans to acquire US F-35 fighter jets, saying it wants “new fighters capable of vertical or short take-off and landing and having stealth characteristics”.

Derek Grossman, a senior defence analyst with Rand Corp, said “Taiwan is certainly interested in acquiring the F-35 for the vertical/short take-off and landing capability it would provide its air force.”

Grossman said Taiwan’s interest in getting F-35s is driven by the accuracy of China’s short-range ballistic missiles, which could target the Taiwan air force’s runways in precision strikes to keep the island’s aircraft grounded.

“If tensions continue to persist in the US-China relationship, it’s conceivable Washington might ramp up arms sales beyond just once or twice a year,” Grossman said.

The Trump administration, however, “could alternatively dangle the prospect of cutting arms sales as a bargaining chip in broader policy discussions with China, whether on North Korea, trade, or the South China Sea”, Grossman said.