VOA Mandarin service's Libo Lui contributed to this report
SINGAPORE — Since the first week acting U.S. Defense Secretary Pat Shanahan took the reins at the Pentagon, he's said his top concern is "China, China, China."
On Saturday, Shanahan told an audience at the annual Shangri-La defense forum that the U.S. would not ignore Chinese behavior, which he says has threatened prosperity in the region.
"It's not about conflict. It's not about building walls. This is about security," Shanahan said.
WATCH: Shanahan Criticizes China's Cyber, South China Sea Behavior
China is infamous for its theft of other nations' military and civilian technology, and the U.S. secretary said he took issue with Beijing's cyber attacks and state-sponsored stealing of intellectual property.
Experts say China has used this theft to narrow the gap between some critical American and Chinese military abilities.
"The kind of advantage that we had against China and the western Pacific during the Cold War is gone for good," Michael O'Hanlon, a senior defense expert with the Brookings Institution, told VOA.
China also has continued to project more military power beyond its borders, most notably by constructing artificial islands in the South China Sea and placing heavy weapons on them to support territorial claims not recognized under international law.
Speaking to reporters Friday, Shanahan called the moves "excessive," saying that while the Chinese "argue that it's defensive, it looks like it's a bit overkill."
Recently, the U.S. has pushed for more international patrols in the South China Sea, including one last month with Japan, India and the Philippines.
Bradley Bowman, of the Foundation for Defense of Democracies, said working with allies to combat Chinese aggression in the region will bolster the U.S. position of protecting international waterways key to global trade.
"I think we need to characterize this conflict for what it is. It's not a conflict between China and the US. It's a conflict with China and the world," he said.
Speaking to VOA at the conference, Rep. Adam Smith, the chairman of the House Armed Services Committee, said China has to some extent "overplayed their hand."
"They are seen as a bully in the region by a lot of people. They encroached on people's territory in a variety of disputes ... and in doing that, those countries have turned to the U.S.," Smith said.
In his speech Saturday, Shanahan stressed that the U.S. doesn't want any nation in the region to have to choose positive relations with one partner over another, but, he said, the world deserved a fair playing field.
U.S. allies at the conference expressed anxiety over rising tensions between the two powers, and as one leader pointed out, many believe that if China and the U.S. won't work together, they risk upending the global system.