US President Donald Trump and South Korean President Moon Jae-in held a half-hour phone discussion on ways to maintain diplomacy with North Korea, Seoul said on Saturday, as Pyongyang warns of waning patience with stalled nuclear talks.

The two leaders agreed that the situation has become ‘severe’ and that ‘dialogue momentum should be maintained to achieve prompt results from denuclearisation negotiations,’ South Korea's presidential Blue House said in a statement.

They also agreed to consult regularly as needed, the statement said.

Tensions are rising ahead of a year-end deadline set by North Korea, which has called on the United States to change its policy of demanding unilateral denuclearisation or leader Kim Jong Un may choose an unspecified ‘new path’.

In a series of statements in recent weeks, senior North Korean officials have warned the United States not to ignore the deadline, and called Washington's offer of more talks a stalling tactic ahead of next year's US election.

Longtime allies, the United States and South Korea are also in negotiations over how much each side will pay to fund the roughly 28,500 US troops stationed in South Korea.

Trump has demanded that Seoul pay as much as $4 billion more per year, according to South Korean officials, and a fourth round of talks ended in Washington on Wednesday with no agreement.

The Blue House said the cost-sharing dispute was not raised during Moon's call with Trump on Saturday.