"Troubling and counterproductive" rhetoric from North Korea is inflaming tensions on the Korean peninsula, but ultimately, "we will judge North Korea by its actions, not its words," the U.S. envoy to North Korea said Friday.

In its latest bout of saber-rattling, North Korea on Friday warned of the possibility of "strong physical counter-measures" against South Korea after the United Nations imposed tougher sanctions against the North earlier this week.

The threat against South Korea came a day after the North said it would carry out a new nuclear test and more long-range rocket launches as part of a new phase of confrontation with the United States.

The statement Friday from North Korea's Committee for Peaceful Reunification of the Fatherland said it would take action against South Korea if it "takes a direct part" in the U.N. sanctions.

The South Korean Unification Ministry declined to comment specifically on the new threats from Pyongyang. It reiterated its stance that North Korea should refrain from further provocations.

"Now is a moment when I think all parties in the six-party process, in particular here, North Korea, should turn their attention to how to peacefully and diplomatically address challenges that concern them," U.S. envoy Glyn Davies said from Beijing.

The two Koreas have technically still been at war since the all-out conflict between them in the 1950s. Smaller scale clashes have occurred since then, most recently in November 2010 when North Korea shelled a South Korean island, killing several people.

South Korea and the United States are often the focus of menacing language from Pyongyang, but the latest U.N. sanctions, a response to a long-range rocket launched last month by the North, appear to have prompted a ratcheting up of the threats.

A displeased Chinese editorial

At the same time, North Korea's strong words and vow to conduct a third nuclear test -- previous ones took place in 2006 and 2009 -- appear to be testing the patience of its main ally, China, which voted in favor of the U.N. sanctions this week.