President Donald Trump's National Security Advisor Robert O'Brien told the Aspen Security Forum that a missile attack by China, which regards Taiwan as a rebel province, would be much too destructive.

An amphibious attack, he said, is a possibility, though at the moment beyond China's capability.

But China could combine that threat with "grey zone" operations, embargos, harassment and other actions to intimidate the island if Taipei does not build up its defense, O'Brien said.

"What we told our Taiwanese friends is knowing all this, whether there's an amphibious landing, a missile attack, a grey zone-type operation, they really need to fortify themselves."

"Taiwan needs to start looking at some asymmetric and anti-access area denial strategies... and really fortify itself in a manner that would deter the Chinese from any sort of amphibious invasion or even a grey zone operation against them," said O'Brien.

China has stepped up pressure against Taiwan over the past year, sending attack and surveillance aircraft into its airspace and ships near its waters.

Earlier this week Beijing released video of a military exercise simulating an invasion featuring missiles and amphibious landings.

At the same time, news reports said Taiwan was close to buying missile systems from the United States.

For decades, Taiwan's security has relied in part on close relations with the United States.

But there is always ambiguity over whether Washington would intervene and defend Taiwan if China attacks.

While China has a large number of missiles pointed at Taiwan, O'Brien said he didn't think Beijing currently wants to attack the island.

"I don't know what they would gain from that," he said.

"If they did that, maybe they would certainly become pariahs internationally for just the wanton destruction of Taiwan."

Noting China's massive naval buildup, though, he said: "Maybe in 10 or 15 years, they'd be in better shape to do it."

Defense News reported this week that the United States is planning to sell several arms systems, including missiles and Reaper drones, to Taiwan.