Washington DC - China, despite its public alliance with Russia, is worried North Korea could be emboldened into starting a regional crisis following President Vladimir Putin's visit, a top US official said Monday.

Deputy Secretary of State Kurt Campbell said that China has indicated in interactions with the US that it is "anxious" after Putin last week signed a defense deal with Pyongyang.

"I think it would be fair to say that China is probably worried that North Korea will be somehow encouraged to take provocative steps that could lead to a crisis in Northeast Asia," Campbell said at the Council on Foreign Relations.

He pointed to the rise in small-scale military incidents by North Korea on its frontier with the South, as well as Pyongyang's "really provocative language" and "absolute clear determination" to avoid diplomacy with the US.

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Russia, the US believes, is looking at greater support for North Korea, including potentially on the nuclear front, Campbell said.

"This is a dangerous set of developments and one that we are watching," Campbell said.

North Korea is under a slew of UN sanctions over its nuclear and missile programs but has found an eager partner in Russia, a veto-wielding Security Council member, by pumping out artillery shells for Moscow's invasion of Ukraine.

Putin signed an agreement with North Korean leader Kim Jong Un that included a pledge for the two countries to come to each other's aid if attacked, a seeming return to a Cold War-era alliance.

China, which has long been the primary partner of Pyongyang but recently has been keeping Kim at arm's length, has said little publicly about Putin's trip.

The US has also accused China of fueling a military buildup by Russia through industrial exports, although not direct arms shipments.

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Campbell said that while China and Russia were united on antagonism to the West, the two powers had different takes not only on North Korea but on Beijing's ties with former Soviet republics in Central Asia.

"I think there will likely, in the future, be a resurgence of tension between Moscow and Beijing," Campbell said.

But in the nearer term, Campbell said, China has gained the upper hand and may be able to source from Russia submarines or other technologies on which Moscow remains more advanced.