With ongoing tensions in the South China Sea, the United States has given one of its key allies an ultimatum.

As Beijing’s influence grows in the Pacific, Washington has gone to extremes to prevent its own decline. In addition to pressuring regional allies to act more aggressively, the Pentagon has conducted its own provocations, performing "freedom of navigation" exercises through Chinese waters.

Now the US expects Australia to choose between Washington and Beijing.

"I think the Australians need to make a choice…it’s very difficult to walk this fine line between balancing the alliance with the United States and the economic engagement with China," said US Army Assistant Chief of Staff Colonel Tom Hanson, according to Asia Times.

"There’s going to have to be a decision as to which one is more of a vital national interest for Australia."

The Pentagon relies on Australian air bases to conduct surveillance flights over Beijing’s artificial islands in the South China Sea. The US has accused China of building the islands to establish an air defense zone, while China maintains it has every right to build within its own territory and that the islands will be used primarily for humanitarian purposes.

Australia, however, also has a strong trading relationship with China. According to the Jerusalem Post, Beijing spent $11.1 billion on Australian assets in 2015.

As a show of loyalty, Hanson suggested that Canberra conduct a freedom of navigation patrol of its own.

"Clearly, China believes that they have an opportunity and they feel empowered to flout that, and a demonstration by Australia would be welcome," he said.

Hanson added that his statements reflect his own opinions and do not represent the official views of the US government.

In response, Canberra gave no indication that it planned to change its positions.

"We are balancing relationships between our largest strategic ally and our largest trading partner with deft diplomacy, consistency and pragmatism," Australian Foreign Minister Julie Bishop said in a statement, according to Asia Times.

A highly disputed region through which roughly $5 trillion in international trade passes annually, most of the South China Sea is claimed by China, though there are overlapping claims by Malaysia, Brunei, Vietnam, Taiwan, and the Philippines.

Neither the United States nor Australia hold any claims in the waterway.