Candidates for the Democratic presidential nomination are speaking out against Donald Trump’s venture into North Korea, the first time a sitting US president has stepped into the country.

The Republican president met with the North Korean leader in the Demilitarized Zone (DMZ) that divides North and South Korea on Sunday, in a meeting that appeared to have been planned on a whim earlier in the week. Joining many critics who called the journey a publicity stunt, nominees for the top Democratic spot voiced negative opinions of the weekend meeting.

“Our President shouldn’t be squandering American influence on photo ops and exchanging love letters with a ruthless dictator,” wrote senator Elizabeth Warren on Twitter on Sunday morning. “Instead, we should be dealing with North Korea through principled diplomacy that promotes US security, defends our allies, and upholds human rights.”

Senator Kamala Harris also used the platform to voice concern, tweeting “This President should take the North Korean nuclear threat and its crimes against humanity seriously. This is not a photo-op. Our security and our values are at stake.”

In an interview on ABC, Senator Bernie Sanders said he had “no problem” with the president meeting with Kim Jong-un but that he didn’t want a meeting to “simply be a photo opportunity.”

“What’s going to happen tomorrow and the next day?” senator Sanders continued. “He has weakened the State Department.”

Candidate Julian Castro also said he did not condemn meeting with adversaries, but did express concerns Mr Trump was raising the profile of a dictator by meeting with the North Korean leader three times with nothing to show for it.

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

North Korea: A road trip on the edge of the Chinese border Show all 20 1 /20 North Korea: A road trip on the edge of the Chinese border North Korea: A road trip on the edge of the Chinese border People have their picture taken with North Korea in the background, in Tumen, China. In the Chinese city of Dandong, signs of neighbouring North Korea are pervasive. REUTERS North Korea: A road trip on the edge of the Chinese border A North Korean woman is photographed from the Chinese side of the Yalu River near the town of Changbai, China as she walks between houses in the North Korean town of Hyesan. A week-long road trip along China's side of its border with North Korea revealed stark contrasts between China's bright towns and North Korea's dark villages. REUTERS North Korea: A road trip on the edge of the Chinese border North Korean souvenirs are displayed for sale on the banks of the Yalu River in Dandong in Liaoning province, China. In the Chinese city of Dandong, signs of neighbouring North Korea are pervasive. Reuters North Korea: A road trip on the edge of the Chinese border A large screen, which faces North Korea, broadcasts propaganda videos on an island on the Yalu River between North Korea and China, in the town of Linjiang in Jilin province, China. A week-long road trip along China's side of its border with North Korea revealed stark contrasts between China's bright towns and North Korea's dark villages. REUTERS North Korea: A road trip on the edge of the Chinese border A piece of clothing is used to make a gap in barbed wire near the closed bridge over the Yalu River on the Chinese side of the border with North Korea between towns of Ji'an and Linjiang, China. A week-long road trip along China's side of its border with North Korea showed places where it is porous. Reuters North Korea: A road trip on the edge of the Chinese border A tourist uses binoculars to look across to North Korea from a tower built on the Chinese side of the border between Russia (L), China (C) and North Korea (R) near the town of Hunchun in China. Reuters North Korea: A road trip on the edge of the Chinese border North Koreans are photographed from the Chinese side of the border as they stand next to the freezing Yalu River near the town of Linjiang, China. On a road trip along China's side of its border with North Korea, reporters found a group of North Koreans diving in the Yalu river who the Chinese locals said were searching for gold. REUTERS North Korea: A road trip on the edge of the Chinese border Women participate in a group dance exercise close to the Yalu River which runs between China and North Korea, in the town of Linjiang in Jilin province, China. A week-long road trip along China's side of its border with North Korea revealed stark contrasts between China's bright towns and North Korea's dark villages. REUTERS North Korea: A road trip on the edge of the Chinese border A sign reading, "Take the initiative to preserve order along the border," stands in a field at the border between China and North Korea just outside Dandong, Liaoning province, China. Reuters North Korea: A road trip on the edge of the Chinese border A couple prepare for a wedding photography session on a boat which takes tourists on sightseeing tours from the Chinese side of the Yalu River, close to the shores of North Korea, near Dandong, Liaoning province, China. In the Chinese city of Dandong, signs of neighbouring North Korea are pervasive. REUTERS North Korea: A road trip on the edge of the Chinese border Tourists travel on a boat taking them from the Chinese side of the Yalu River for sightseeing close to the shores of North Korea, near Dandong, Liaoning province, China. In the Chinese city of Dandong, signs of neighbouring North Korea are pervasive. REUTERS North Korea: A road trip on the edge of the Chinese border North Koreans are photographed from the Chinese side of the border as they stand next to the freezing Yalu River near the town of Linjiang, China. On a road trip along China's side of its border with North Korea, reporters found a group of North Koreans diving in the Yalu river who the Chinese locals said were searching for gold. REUTERS North Korea: A road trip on the edge of the Chinese border Women walk across a bridge from an island on the Yalu River, dividing North Korea and China, in the town of Linjiang in Jilin province, China. A week-long road trip along China's side of its border with North Korea revealed stark contrasts between China's bright towns and North Korea's dark villages. REUTERS North Korea: A road trip on the edge of the Chinese border A North Korean soldier and locals are photographed from the Chinese side of the border north of Dandong, China as they stand on the banks of the Yalu River, north of Sinuiju, North Korea. A week-long road trip along China's side of its border with North Korea revealed stark contrasts between China's bright towns and North Korea's dark villages. REUTERS North Korea: A road trip on the edge of the Chinese border North Korean girls are photographed from the Chinese side of the border as they collect water from the frozen Yalu River near Linjiang, China. A week-long road trip along China's side of its border with North Korea revealed stark contrasts between China's bright towns and North Korea's dark villages. REUTERS North Korea: A road trip on the edge of the Chinese border A man looks through binoculars towards North Korea on the Broken Bridge over the Yalu River that connects the North Korean town of Sinuiju and Dandong in Liaoning Province, China. In the Chinese city of Dandong, signs of neighbouring North Korea are pervasive. REUTERS North Korea: A road trip on the edge of the Chinese border North Koreans are photographed from the Chinese side of the border as they stand next to the freezing Yalu River near the town of Linjiang, China. On a road trip along China's side of its border with North Korea, reporters found a group of North Koreans diving in the Yalu river who the Chinese locals said were searching for gold. REUTERS North Korea: A road trip on the edge of the Chinese border North Korean fishermen are seen as a Chinese flag flutters from the Broken Bridge as the sun sets over the Yalu River between the North Korean town of Sinuiju and Dandong in Liaoning Province, China. In the Chinese city of Dandong, signs of neighbouring North Korea are pervasive. REUTERS North Korea: A road trip on the edge of the Chinese border North Koreans are photographed from the Chinese side of the border as they stand in the freezing Yalu River near the town of Linjiang, China. On a road trip along China's side of its border with North Korea, reporters found a group of North Koreans diving in the Yalu river who the Chinese locals said were searching for gold. REUTERS North Korea: A road trip on the edge of the Chinese border Women, photographed from the Chinese side of the border, are seen through binoculars fixed on a building on the Chinese side of the border for sightseers to look across to North Korea, as they cross the bridge from Namyang in North Korea towards the town of Tumen in China, Reuters

A spokesman for former vice president Joe Biden said the president was "coddling" dictators at the expense of U.S. national security.

Senator Amy Klobuchar, also running for the candidacy, compared the move to a moment between neighbours.

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