"The US is going to make sure that we protect our interests there", Spicer said.
Chinese foreign ministry spokesperson, Hua Chunying, responded on Tuesday saying the United States should tread carefully "to avoid harming the peace and stability of the South China Sea".
"China urges the US side to respect facts and speak and act with caution so as not to impair peace and stability in the South China Sea region", Hua said at a regular briefing.
U.S. President Donald Trump speaks during the Inaugural Law Enforcement Officers and First Responders Reception in the Blue Room of the White House in Washington, U.S., January 22, 2017.
The story South China Sea: China quiet after Secretary of State nominee Rex Tillerson's tough talk first appeared on The Sydney Morning Herald.
During his Senate confirmation hearing, Tillerson suggested the United States could block China's access to the islands, which the Chinese state tabloid said would be an "illegal act" that could lead to a major military conflict.
Tillerson's comment drew fierce rebuke from Chinese state-run media and US-based China watchers as well, with Chinese media saying that "unless Washington plans to wage a large-scale war in the South China Sea, any other approaches to prevent Chinese access to the islands will be foolish".
Mr Spicer's comments, and those of Mr Tillerson, suggest that the USA might be ready to intervene militarily in the region, which would nearly certainly lead to confrontation with China, a rising military power in the region.
In recent years, China has angered its neighbours, including Malaysia, Brunei, the Philippines, Vietnam and self-ruled Taiwan, by its aggressive claims to territories in the maritime region, through which €4.75 trillion in trade is shipped every year.
Washington-based South China Sea expert Mira Rapp-Hooper described the US' threats to China's access to the South China Sea as "incredible", adding that they have no basis in global law.
He said that almost $4.5tn (£3.4tn) in trade passes each year through the South China Sea. And while a July U.N. ruling rejected China's claim to nearly all of the vast maritime region, Beijing has since mended fences with two big claimants in the hotly contested waters, the Philippines and Vietnam.
Experts earlier warned that any attempt by the USA to blockade the islands risks triggering a risky military confrontation between the world's two biggest economies.