South Korea on Monday joined the United States in insisting that rival North Korea take "concrete" steps towards abandoning its nuclear weapons if it genuinely wants to end its international isolation.
The South's comment came a day after Pyongyang sought to circumvent Seoul by proposing direct talks with Washington.
The move was widely seen as an effort to drive a wedge between the United States and its South Korean ally after planned North-South talks were cancelled last week in a row over protocol.
The US response was that the North would not be allowed to "talk its way out" of UN sanctions, and would have to provide some tangible proof of its willingness to curb its nuclear programme.
"We'll judge them by their actions, not by the nice words that we heard," White House Chief of Staff Denis McDonough said Sunday.
The South's Unification Ministry, which deals with inter-Korean relations, made it clear that Seoul and Washington were firmly on the same page.
"I'd like to explain our stance by repeating the comments from the White House ... that the window of dialogue is open but that the North should take concrete steps first," said ministry spokesman Kim Hyung-Seok.
South Korean President Park Geun-Hye discussed the North Korean issue with US President Barack Obama by phone on Monday, her spokeswoman said.
During the 20-minute call, Obama briefed Park on his recent summit with Chinese President Xi Jinping, when both leaders agreed to work together on the denuclearisation of the Korean peninsula.
Park and Xi are due to hold their own summit in Beijing on June 27.
North Korea has embarked on something of a diplomatic offensive of its own after months of heightened military tensions on the Korean peninsula, during which it had threatened nuclear strikes against the South and the US.
In May, it received a top-level aide to Japan's prime minister, and leader Kim Jong-Un sent his personal envoy to Beijing for talks with the Chinese leadership.
Then it proposed high-level talks with South Korea which were called off at the last minute after the two sides argued over the rank of their respective chief delegates.
Sunday's proposal for direct talks with Washington insisted there could be no "pre-conditions" -- an optimistic requirement given the US insistence that the North first demonstrate its sincerity about denuclearisation.
South Korean media dismissed Pyongyang's talks offer as a worn-out strategy to divide Seoul and Washington, with an editorial in the JoongAng Daily calling it "North Korea's indecent proposal".
Last week, Glyn Davies, the US pointman on North Korea, repeated calls for the North to take steps to end its nuclear programme and said the recent surge in tensions on the Korean peninsula had reinforced US hesitancy to engage again.
The last senior-level talks between North Korea and the United States in February 2012 resulted in a deal for supplies of US food aid in exchange for a moratorium on missile and nuclear tests.
The agreement collapsed almost immediately when the North unsuccessfully launched a long-range rocket the following month.