The run-in near Japan's Okinawa islands comes after Japanese Prime Minister Sanae Takaichi triggered a dispute with Beijing in November with her remarks on how Tokyo might react to a hypothetical Chinese attack on Taiwan.
China claims democratically governed Taiwan and has not ruled out using force to take control of the island, which sits about 100km from Japanese territory and is surrounded by sea lanes on which Tokyo relies.
"China's actions are not conducive to regional peace and stability," a State Department spokesperson said late on Tuesday, referring to the radar incident.
"The US-Japan Alliance is stronger and more united than ever. Our commitment to our ally Japan is unwavering, and we are in close contact on this and other issues."
Japanese Chief Cabinet Secretary Minoru Kihara welcomed the comments, saying they "demonstrate the strong US-Japan alliance".
Speaking in Beijing, Chinese foreign ministry spokesman Guo Jiakun avoided direct criticism of the US remarks, reiterating that China's training and exercise activities were in line with international law and conducted safely in a restrained manner.
"We hope that the international community can tell right from wrong and not be hoodwinked by the Japanese side. Japan's allies, in particular, should heighten their vigilance and not be manipulated by Japan," he said.
Japan late on Tuesday scrambled jets to monitor Russian and Chinese air forces conducting joint patrols around the country.
On Wednesday, Japan's coast guard said its ships spotted four Chinese coast guard vessels near the disputed but Japan-administered Senkaku islands in the East China Sea.
China's coast guard said it was carrying out a "legal" operation to safeguard the country's rights and interests.
The Chinese fighter jets aiming their radars at the Japanese planes on Saturday was the most serious run-in between the East Asian militaries in years.
Such moves are seen as a threatening step because they signal a potential attack and might force the targeted plane to take evasive action.
Tokyo blasted the moves as dangerous.
Beijing, however, said the Japanese aircraft had repeatedly approached and disrupted the Chinese navy as it was conducting previously announced carrier-based flight training east of the Miyako Strait.
Speaking to reporters in Taipei on Wednesday, Taiwan President Lai Ching-te said China's drills were "very inappropriate behaviour".
"We also call upon China to demonstrate the responsibility befitting a major power. Peace is priceless; war has no winners. Peace must be fostered by all parties, and China shares this responsibility," he said.
Relations between Asia's two largest economies have soured sharply since Takaichi told parliament in November that a Chinese attack on Taiwan could amount to a "survival-threatening situation" and trigger a potential military response from Tokyo.
Beijing has demanded she retract the remarks, accusing Tokyo of threatening it militarily and advising its citizens not to travel to Japan.
