The U.S. has flown two B-1B bombers over the South China Sea, the air force said Friday, part of operations intended to demonstrate Washington’s commitment to freedom of navigation in the contested waters and airspace.

China, which claims almost all of the strategic waterway, slammed the move, with Foreign Ministry spokesman Geng Shuang maintaining that freedom of navigation or overflight in the East and South China seas was not a problem.

Experts say technological advances by China and and growing nationalism there, combined with ramped-up moves by Washington in the South China Sea could mean dangerous, close-proximity encounters in the air and sea will become a more common occurrence.

Collin Koh, a researcher at the S. Rajaratnam School of International Studies in Singapore, said China’s more advanced aircraft and its airstrips on the artificial islands in the Spratlys give it more latitude to respond promptly and firmly.

“These are the enablers,” Koh said. “What is of concern is that China’s disdain for foreign military activities in and over waters it claims. It wasn’t in a position back then to respond more resolutely to those activities because of the aforementioned capability limitations. Now that these are improved, Beijing would be more inclined to challenge these foreign activities.”