China has hit back at the US in a row over marine territorial claims.
Foreign ministry spokeswoman Hua Chunying alluded to the US when she blamed "some countries" for flexing "their military muscles again and again" in the South China Sea.
Last week, US officials said they were considering sailing warships in an area around the Spratly island chain which China claims as territory.
It has sparked tit-for-tat warnings between the two powers.
China has been worrying its neighbours - and the US - by enlarging the series of tiny islands, reports the BBC's China analyst Michael Bristow.
Washington believes Beijing is constructing military facilities, designed to reinforce its disputed claim to most of the region - a major shipping zone.
China says its work is legal and needed to safeguard its sovereignty.
'Make no mistake'
The row began when US officials said they were considering sending warships inside the 12-nautical-mile zones that China claims as territory around the Spratlys.
That sparked strong words from China, with Ms Hua warning: "We will never allow any country to violate China's territorial waters and airspace in the Spratly Islands, in the name of protecting freedom of navigation and overflight."
On Tuesday, US Defence Secretary Ash Carter expressed "strong concerns" over island-building, and defended Washington's plans.
"Make no mistake, the United States will fly, sail and operate wherever international law allows, as we do around the world, and the South China Sea will not be an exception," he said at a news conference with the Australian foreign and defence ministers.
"We will do that in the time and places of our choosing," he added, according to Reuters news agency.
The dispute in the South China Sea is over the sovereignty of ocean areas, and the Paracels and the Spratlys - two island chains claimed in whole or in part by a number of countries.
Alongside the fully fledged islands, there are dozens of rocky outcrops, atolls, sandbanks and reefs, such as the Scarborough Shoal.
Although largely uninhabited, the Paracels and the Spratlys may have reserves of natural resources around them.
The sea is also a major shipping route and home to fishing grounds that supply the livelihoods of people across the region.
That in turn triggered Wednesday's rebuke from China.
Asked about Mr Carter's comments, Ms Hua said: "I want to point out that some countries, in a region far from their own lands, have deployed offensive weaponry on a large scale and flexed their military muscles again and again in the South China Sea.
"This is the biggest factor in the militarisation of the South China Sea. We hope the relevant countries cease hyping up the South China Sea issue and scrupulously abide by their promises not to take a position on the territorial disputes," she said, according to Reuters.
The US might have mounted sea patrols in this area, but not for several years, our analyst says - and not since China began its massive building programme in the South China Sea.
A US military plane that flew near one of the islands in May was warned off - eight times.
The US now has to decide whether to send in its ships and risk confrontation, or back down and look weak, our analyst says.
Earlier this week, Vietnam criticised China's completion of two lighthouses in the South China Sea's Spratly islands, arguing they violated Vietnam's sovereignty.
Beijing maintains the lighthouses will improve navigation.