Both Taiwan and China claim most of the South China Sea as their own territory, but Taiwan only controls one main islet in the contested Spratly Islands deep in the southern part of the sea called Itu Aba, which Taiwan refers to as Taiping island.

Some lawmakers from both the ruling and main opposition parties have called on Tsai to visit Itu Aba before she steps down in May to assert Taiwan's sovereignty and view a newly renovated harbour that can take larger ships.

Both her predecessors visited the island but she has yet to do so while in office.

Taiwan National Security Bureau Director-General Tsai Ming-yen said the South China Sea had been highly militarised, and the government must also consider how the international community would view such a trip.

"Many countries' aircraft and ships are interfered with by relevant countries when passing through," he told reporters at parliament.

"Therefore, it is necessary to assess whether the president's aircraft's flight safety can be maintained during the 1,600 km (994 miles) flight. If the two problems cannot be resolved, it is not recommended that the president go to Taiping island at this stage."

China has reclaimed land and build air bases and other military facilities on some of its islands including those close to Itu Aba, and regularly objects to U.S. navy ships and military aircraft operating nearby in what Washington calls freedom of navigation operations.

Itu Aba has a runway long enough to take military re-supply flights from Taiwan, which take around four hours, but is lightly defended compared to the nearby Chinese-controlled islands. Chinese forces generally leave Itu Aba alone.

Taiwan also controls the Pratas Islands in the northern part of the South China Sea, and both China's air force and navy regularly operate nearby to assert Beijing's territorial claims over Taiwan, which the government in Taipei rejects.

Vietnam, Malaysia and Brunei claim other parts of the South China Sea in dispute with both China and Taiwan.

(Reporting by Ben Blanchard; Editing by Stephen Coates)