WASHINGTON (Reuters) - Defense spending in Taiwan has not kept pace with the threat posed by China and should be increased, a senior U.S. defense official said on Tuesday, days after U.S. President-elect Donald Trump touched off a storm by questioning American policy over the island.

Taiwan's President Tsai Ing-wen speaks on the phone with U.S. president-elect Donald Trump at her office in Taipei, Taiwan, in this handout photo made available December 3, 2016. Taiwan Presidential Office/Handout via REUTERS/File Photo

Deputy Assistant Secretary of Defense Abraham Denmark said the Obama administration’s “One China” policy remained unchanged, but he could not predict Trump’s intentions when he takes office on Jan. 20.

Trump set off a diplomatic firestorm over the weekend when he questioned why the United States should be bound by the long-standing policy under which Washington recognizes Beijing rather than Taiwan, which China considers a renegade province.

This followed an earlier Chinese protest over Trump’s telephone conversation with Taiwanese President Tsai Ing-wen on Dec. 2, the first involving a U.S. President-elect or president since 1979.

Some U.S. analysts warn that Trump could provoke a military confrontation if he presses the Taiwan issue too far.

Denmark told the Project 2049 Forum in Washington that the main focus of China’s military modernization program was to achieve reunification with Taiwan, by force if necessary.

“This makes it incumbent on Taiwan to prepare and invest in capabilities to deter aggression and mount an effective defense if deterrence fails,” he said.

“Defense resourcing is critical,” he said. “Taiwan’s defense budget has not kept pace with the threat developments and should be increased.”

The United States is the main political ally and sole arms supplier to diplomatically isolated Taiwan.

Denmark said the administration had notified Congress of more than $14 billion of arms sales to Taiwan since 2010 but declined to say whether any further sales were possible before President Barack Obama leaves office.

He said the United States was committed under the Taiwan Relations Act to ensuring it had the capability to defend Taiwan. But he stressed that any decision to do so was up to the resident and the primary responsibility remained with Taiwan itself.

Taiwan has been working to develop its own defense equipment and last year allocated an initial budget for a submarine program, but technology transfer is critical to the success of such projects and it has yet to receive key foreign technological support.

Trump adviser and China hawk Peter Navarro, who has produced books and multipart television documentaries warning of the dangers of China’s rise, has suggested stepped up engagement with Taiwan, including assistance with its submarine development program.

The Obama administration notified Congress a year ago of a $1.83 billion arms sale package for Taiwan, including two frigates, antitank missiles, amphibious assault vehicles and other equipment, drawing an angry response from China.