The US national security adviser warned China on Wednesday against any attempt to retake Taiwan by force, saying amphibious landings were notoriously difficult and there was a lot of ambiguity about how the United States would respond.

Robert O’Brien told an event at the University of Nevada in Las Vegas that China was engaged in a massive naval buildup probably not seen since Germany’s attempt to compete with Britain’s Royal Navy prior to World War One.

“Part of that is to give them the ability to push us back out of the Western Pacific, and allow them to engage in an amphibious landing in Taiwan,” he said.

O’Brien was referring to a long-standing US policy of “strategic ambiguity” on the question of whether it would intervene to protect Taiwan, which China considers a its province and has vowed to reunite with the mainland, by force if necessary.

The United States is required by law to provide Taiwan with the means to defend itself, but it has not made clear whether it would intervene militarily in the event of a Chinese attack, something that would likely lead to a much broader conflict with Beijing.