South Korean President Moon Jae-in remains optimistic about the resumption of talks between the United States and North Korea, saying that it is too early to feel disappointed as neither side has closed the door to dialogue.

However, he warned that "not much time is left" to strike a deal as the US enters full election mode this year - presidential elections are due in November - and is bogged down by a feud with nuclear-armed Iran.

Speaking at his first press conference of the year, President Moon yesterday also stressed the need to find realistic solutions to improve stalled inter-Korean ties so as to create a virtuous cycle that could also help advance US-North Korea talks.

"We should do all we can in widening inter-Korean cooperation, so as to advance US-North Korea talks and win international support for the easing of sanctions on North Korea," he said.

Inter-Korean ties have stalled since US-North Korea talks broke down in Hanoi last February over a lack of agreement on what denuclearisation steps Pyongyang should take in exchange for concessions from economic sanctions.

North Korean leader Kim Jong Un on Dec 31 declared the end of a nuclear moratorium and warned about the unveiling of a "new strategic weapon".

While refraining from conducting missile tests, Pyongyang has continued to exert pressure on the US and insist that Washington must agree to its demands for sanctions relief.

President Moon yesterday described as a good sign the fact that US President Donald Trump made the effort to send birthday greetings twice to Mr Kim last week - via a personal letter and asking South Korea to pass the message.

The South Korean President noted that the North responded immediately to Mr Trump's good wishes, in a move that underscored the friendly relations between the two leaders.

"Despite facing turbulent issues, President Trump still sent birthday greetings to Chairman Kim. This means he regards Chairman Kim as an important counterpart in foreign affairs," said Mr Moon.

"Dialogue may be stalled but the two leaders continue to trust each other... the door to dialogue remains open," the South Korean leader added.

China can also play a crucial role in resolving North Korea's nuclear issue, said President Moon. China is North Korea's only ally and economic lifeline.

He noted that Chinese President Xi Jinping and Premier Li Keqiang are scheduled to visit Seoul separately this year - Mr Xi for a state visit and Mr Li will be attending a trilateral summit with Japan and South Korea.

"This will be an opportunity for China-South Korea relations to make a quantum leap," Mr Moon said.

US national security adviser Robert O'Brien on Sunday said that Washington has reached out to Pyongyang in a bid to resume talks. But the North may not respond.

President Moon seemed undeterred. He said yesterday that his government would continue to push to expand inter-Korean cooperation with an "optimistic" view of future prospects.

"There are things we can do within the limits of international sanctions, such as border management and individual tourism programmes. We also can collaborate in sports, such as doing a joint march-in at the Tokyo Olympics."