President Xi Jinping has greater control of the military than his predecessor did and that increases the need for a strong White House relationship with him, former US defence secretary Robert Gates says.
Gates, 70, said in an interview on Monday that former president Hu Jintao "did not have strong control" of the People's Liberation Army.
The "best example", Gates said, was China's roll-out of its stealthy J-20 fighter jet during a visit he made in January 2010. The event seemed to catch Hu unawares, Gates said, recounting a story in his memoir Duty, which went on sale yesterday.
China posed few crises for Gates during his 4-1/2 years as defence secretary.
Tensions have grown since the Obama administration announced plans to step up military and political engagement in the Pacific, a strategy announced after Gates left office, and China stepped up its territorial claims off its coast.
Xi's stronger control is "both a good-news and a bad-news story", Gates said.
"Before, when the Chinese did something aggressive or risky, you could say, 'That's the PLA acting on their own'. Now, when they do something like declare a new air defence identification zone [in the East China Sea in November] you've got to assume President Xi approved that and is on board" and not that "this is just the PLA misbehaving or strutting its stuff", he said.
Gates said China's declaration was a "real provocation in the current environment" and "meant to send a signal".
Xi's control of the PLA meant his relationship with President Barack Obama "matters a great deal, and managing this relationship is going to be a big challenge for both countries".
He wrote China's military was "designed to keep US air and naval assets well east of the South China Sea and Taiwan".
Last month a US Navy guided-missile cruiser had a confrontation with a Chinese military ship in the South China Sea, underscoring the rising tensions in the region over China's new air defence zone.
Defence Secretary Chuck Hagel called China's behaviour "irresponsible".
The US Pacific presence was being countered by a growing Chinese navy that "while far inferior" to the US globally, "could be a serious problem for us in Northeast and Southeast Asia" as it developed "highly accurate cruise and ballistic missile, diesel and nuclear submarines and stealthy fighters", Gates wrote.
"Beijing learned from the Soviet experience, I believe, and has no intention of matching us ship-for-ship, tank-for- tank", and "thereby draining China financially in a no-holds barred arms race", he wrote.
Watch: Xi Jinping featured in a Chinese animation