The United States remains "always vigilant" about North Korea's potential steps against South Korea that might go beyond its sending of trash-filled balloons to the South, a senior US diplomat said Wednesday.

Deputy Secretary of State Kurt Campbell made the remarks amid tensions heightened by the North's recent floating of disturbing balloons across the inter-Korean border and the South's short-lived resumption of frontline loudspeaker propaganda broadcasts.

"We're always vigilant about potential steps that go beyond simply sending trash to the other side," Campbell said during a forum hosted by the Washington-based Stimson Center.

"We believe that South Korea is appropriately prepared for a number of potential provocations, and we are joined in the closest possible partnership with Seoul in this set of endeavors," he added.

The North has sent waste-loaded balloons to the South, taking issue with South Korean activists' sending of anti-Pyongyang leaflets to the North. In response, the South resumed propaganda broadcasts near the border on Sunday, but decided to halt the broadcasts the following day.

Asked about Russian President Vladimir Putin's plan to visit Pyongyang in the near future, Campbell refused to comment, but expressed concerns about the North's arms transfers to Russia and what Pyongyang might get from Moscow in return.

"We also have to recognize that there is the other part of this relationship, and that means what is Russia going to provide in exchange to North Korea? Hard currency? Is it energy? Is it capabilities that allowed them to advance their nuclear and missile products?" he said.

"We don't know, but we're concerned by that and watching carefully, and I think it would be fair to say that the entire scope of the Russia-North Korea relationship is something that we follow carefully and are concerned by."

Washington has revealed that Pyongyang has shipped over 10,000 containers of munitions or munition-related materials to Russia since September, in addition to its missile shipments.

In return, the North has been seeking assistance from Moscow, including fighter aircraft, surface-to-air missiles, armored vehicles and ballistic missile production equipment, according to US officials.

The deputy secretary underscored that Washington is watching "carefully" what is taking place in Pyongyang.

"There are some changes in their policy approaches, and those are being studied carefully," he said without elaboration. (Yonhap)