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 United States 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 United States.

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

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.

Support for Philippines

Putin's trip to Pyongyang has rattled South Korea, which normalized relations with Moscow after the fall of the Soviet Union and said it will reconsider its ban on supplying weapons to Ukraine.

Putin's disruption comes at a time when Chinese President Xi Jinping has largely sought to lower the temperature with the United States, including through diplomacy with President Joe Biden.

One exception has been over the Philippines, a treaty-bound US ally, which has seen escalating confrontations as Beijing pushes its claims in dispute-rife waters.

In the latest clash, a Filipino soldier lost a finger.

Campbell, asked what actions by Beijing would trigger the treaty, said only that the United States has had "close consultations" with the Philippines about it.

"Our senior officials have been very clear about what circumstances we would consider that the treaty between the United States and the Philippines would come into play," Campbell said, declining to elaborate further.

He said the United States has also taken unspecified "private steps" to address tensions and that the Philippines wanted to be "cautious" in approaching China.

"They do not seek a crisis with China. They are seeking dialogue," he said.