WASHINGTON (REUTERS) - Candidates running for the 2020 United States Democratic presidential nomination on Sunday (June 30) criticised President Donald Trump's latest overture to North Korean leader Kim Jong Un, saying the leaders' meeting lacked substance and elevated a ruthless dictator.

The Republican Trump became the first sitting US president to step into North Korea on Sunday, drawing on his penchant for showmanship and surprise to pull off talks with Mr Kim in the Demilitarised Zone that divides the two Koreas.

The meeting, at Mr Trump's last-minute invitation, drew praise from some including Pope Francis as a step towards peace. Critics called it a publicity stunt and said Mr Trump wasted an important symbolic move when there has been little sign that North Korea has taken meaningful steps towards denuclearisation.

A number of Democrats vying to replace Mr Trump in the White House in the 2020 election said there was nothing wrong with talking to US adversaries including Mr Kim. This round of talks, however, should have followed intense preparations and substantive progress by North Korea on the nuclear issue, they said.

"Our President shouldn't be squandering American influence on photo ops and exchanging love letters with a ruthless dictator," Senator Elizabeth Warren said in a Twitter post.

Her 2020 rival and Senate colleague Bernie Sanders did not fault Mr Trump for meeting Mr Kim.

"But I don't want it simply to be a photo opportunity, the whole world's media was attracted there," he said. "What's going to happen tomorrow and the next day?"

Mr Trump has weakened the State Department at a time when Washington needs to move forward diplomatically towards peace, he said.

While there is nothing wrong with talking to adversaries, Mr Trump has raised the profile of a dictator by meeting Mr Kim three times now with nothing to show for it, according to Mr Julian Castro, US housing secretary under president Barack Obama, and former US Representative Beto O'Rourke.

North Korea has not lived up to commitments made in previous summits with Mr Trump and intense preparations should be have been made before the leaders met again to ensure progress, they said.

"He's doing it backward," Mr Castro said.

A spokesman for former vice-president Joe Biden said Mr Trump was "coddling" dictators at the expense of US national security.

North Korea launched missiles into the sea just last month, Senator Amy Klobuchar noted, showing the need for the US to go into such talks armed with a clear mission and clear goals.

"It is not as easy as just going and bringing a hot dish over the fence to the dictator next door," she said.

The candidates spoke on CNN's State Of The Union, ABC's This Week and CBS' Face The Nation.