U.S. Deputy Secretary of State Kurt Campbell said that the United States is “always vigilant” about potential steps North Korea could take against South Korea that go beyond the launch of trash-carrying balloons.

Campbell made the remarks on Wednesday during a forum hosted by the Washington-based Stimson Center, when asked about the possibility of North Korea engaging in provocations more serious than the balloon launches. 

The senior U.S. official said that the U.S. is carefully watching what is taking place in Pyongyang, adding there have been some changes in the North’s policy approaches, and those are being studied carefully. 

Campbell also said that he believes South Korea is appropriately prepared for a number of potential provocations, and Washington and Seoul are in the closest possible partnership in this set of endeavors. 

Refusing to comment on reports on Russian President Vladimir Putin’s plans to visit North Korea, Campbell expressed concerns about closer military ties and arms transfers between the two nations. 

He said that the U.S. doesn’t know whether Russia will provide hard currency, energy or share their capabilities to help the North advance its nuclear and missile programs in return for the North’s arms transfers, but it is concerned by the possibility and watching carefully.