North Korea fired two short-range missiles in a fresh bid to get the United States to return to negotiations, just as top US nuclear envoy Stephen Biegun began talks with his counterparts in Seoul yesterday.

The missiles were fired within a span of 20 minutes from north-western Kusong city and flew 270km and 420km respectively before falling into the East Sea.

The launch, which came as Seoul was making plans to send food aid across the border, is the second low-level provocation since last Saturday, when Pyongyang fired several rockets and at least one short-range ballistic missile into the East Sea as part of what it called a "regular, self-defensive" drill.

South Korean President Moon Jae-in, speaking on TV yesterday evening, said the North appeared to be expressing displeasure with the collapse of the summit between its leader Kim Jong Un and United States President Donald Trump in Hanoi in February, and it was trying to exert pressure on the US to resume dialogue.

South Korea's presidential Blue House voiced "grave concern" about the launch in a statement, saying it would not help efforts to improve inter-Korea ties and ease military tensions on the Korean peninsula.

Pyongyang has ramped up pressure on Washington since Mr Trump walked away from Mr Kim's demand in Hanoi to ease five major economic sanctions in return for the dismantlement of the regime's main Yongbyon nuclear facility.

The two sides also could not see eye to eye on the US demand for the North to submit a list of its nuclear inventory, a necessary next step in the denuclearisation process.

Mr Biegun is in Seoul to explore ways to resume stalled talks with Pyongyang. He met South Korea's nuclear envoy Lee Do-hoon over breakfast yesterday and is scheduled to meet Foreign Minister Kang Kyung-wha this morning.

South Korea, in trying to bring the two sides back to the negotiating table, is planning to send humanitarian aid to the North, with blessings from the US.

Mr Moon hinted at sending rice last night, saying he will discuss the plan with various political parties.

The United Nations said last week that 10.1 million people in North Korea are facing severe food shortages as agricultural output dropped to the lowest level in a decade. As a result, the regime would require 1.36 million tonnes of external food aid.

Some experts noted that yesterday's missile launch signalled the North's growing frustration and that it wanted more than food aid.

"The North has clearly shown that what it wants is something bigger, that it would not waver under the prospect of food aid, and that it wants a dramatic shift in (the US') hardline stance in the negotiations," Professor Park Won-gon of Handong University was cited as saying in The Korea Herald newspaper.

Clark University's Professor Srinivasan Sitaraman, a North Korea expert, said: "Clearly this is an effort by Kim Jong Un to put pressure on the Trump administration to see if they would give them the sanctions relief."

Analysts also warned that North Korea may continue with low-level provocations to remind the world it can still pose a serious threat until a concession on sanctions is given.

Dr Shin Chang-hoon of the Korea Institute for Maritime Strategy said yesterday's launch shows "North Korea will not get back to negotiations unless the US changes its written position on denuclearisation".