Despite the optimism that preceded the second summit between U.S. President Donald Trump and North Korean leader Kim Jong-un, the two were unable to reach a joint agreement as initially scheduled and the talks ended in a deadlock.

Some summit observers have pointed out the factors behind the leaders being unable to reach an agreement this time.

There was "not enough time or staff" to work out a deal, tweeted Joe Cirincione, president of the Ploughshares Peace Foundation.

Joseph Yun, former senior diplomat, also told CNN that the summit evidenced a "lack of preparation."

Yun said he had attended many summits and usually they take a lot of work at different levels and in fact, "an agreement is a foregone conclusion."

It was clear that little preparation had been made for the Hanoi summit, he said.

Comparing it with the summit in Singapore, where the two leaders met for the first time in June last year, Yun said that event at least laid a foundation and made the world "look at something."

President Trump left Hanoi Thursday afternoon after landing in the Vietnamese capital city on Tuesday for the much-awaited summit. He left sooner than planned and looked sadder than when he arrived.

Before his departure, Trump said it was disagreement on the lifting of sanctions that had been a stumbling block.

"Basically, they wanted the sanctions lifted in their entirety, and we couldn't do that," he said.

"We want to give up the sanctions because that country has so much economic potential. But only when they agree to give up their nuclear (program)," he said.

U.S. Secretary of State Mike Pompeo said: "We asked him [Kim Jong-un] to do more but he was not prepared for that. I wish we could do more."

Trump had repeatedly said before and during the summit that he was in "no rush" to make North Korea denuclearize.

"Speed is not that important to me," he said, adding that he appreciates that North Korea had stopped carrying out nuclear tests.

"What important is that we reach the right deal," he told reporters Thursday morning before meeting with Kim.

Akira Kawasaki of the International Committee to Abolish Nuclear Weapons’ International Steering Group told CNN: "It is little wonder these negotiations broke down after Trump has spent more time in office blowing up nuclear treaties than building them."

He said that in order for North Korea to denuclearize, it needs a real plan with the participation of the international community and treaties like the Treaty on the Prohibition of Nuclear Weapons, the first legally binding international agreement to comprehensively prohibit nuclear weapons, with the eventual goal of total elimination.

Van Jackson, author of "On the Brink: Trump, Kim, and the Threat of Nuclear War," told CNN that the U.S. should have waited to hold the summit until Steve Biegun, U.S. special representative for North Korea, made progress in negotiations.

Trump said his parting with Kim "was very friendly," holding out hope for the future.

"This wasn't a walk away like you get up and walk out. The relationship [with Kim] is good."

"I want to keep the relationship, we'll see what will happen over the next period of time. Chairman Kim told me last night that he will not be doing any nuclear tests and I trust him.

"You always have to prepare to walk away. We actually had papers to sign but it was not appropriate. I don't want to do it quick, I want to do it right," Trump said when asked if it had been premature to hold the summit.

Both leaders had traveled a long distance for their summit. Trump flew around the world for the meeting while Kim traveled more than 3,000 kilometer (1,870 miles) on a train to Vietnam.

Some experts said they were worried that all this efforts had not amounted to anything.

A joint statement signing ceremony had been scheduled for today after leaders had a working lunch. Neither event happened, and both left the summit venue without signing anything.

Ankit Panda with the Federation of American Scientists warned on Twitter that the White House's expectation of further talks "does not have to be a perception shared in North Korea."

"Kim may have left irate, for all we know. He may have no intention of continuing this."