Donald Trump has said he is planning to meet North Korea’s Kim Jong-un later this year even as the reclusive leader’s younger sister cast doubt on their communications and dampened hopes for a diplomatic breakthrough between the two leaders.

Trump, who also claimed North Korea had 57 nuclear weapons, had suggested he and Kim were holding secret negotiations on Tuesday after announcing the US would downgrade its participation in military exercises held by South Korea, a US ally. Asked whether Kim had responded to his request for a conversation, Trump said: “Yeah, he has.”

The Wall Street Journal reported on Tuesday that Trump was pushing aides to set up an in-person meeting with Kim, potentially during a trip to Asia in November for the Asia-Pacific Economic Cooperation summit in Shenzhen, China.

But on Wednesday Kim Yo-jong, the powerful sister of North Korea’s supreme leader, said that she knew “absolutely nothing” about those supposed communications, and said that while Trump and Kim had an “excellent” relationship, the US remained an enemy of Pyongyang.

“The United States’ hostile policy toward the DPRK has not changed, and the United States and its followers are vigorously carrying out hostile military activities against the DPRK even at this very moment,” she said in a statement carried by North Korean state media.

Trump was questioned further about his talks with Kim during a press gaggle in front of Marine One on Wednesday. He declined to say whether or not he was in direct contact with Kim, but asked whether he was planning to meet Kim later this year, he said: “Yeah, I will be.”

“I know Kim Jong-un very well and he’s gonna be fine,” Trump said. “As long as we have a smart president, he’s gonna be fine. If we have a dumb president, then he’s probably not going to be so fine.”

Trump also made the unusual disclosure that North Korea had 57 “very powerful nuclear weapons”, revealing exact intelligence that may not ordinarily be made public. He added that Iran should not be allowed to possess a nuclear weapon.

In her statement, Kim’s sister lauded the relationship between the North Korean leader and Trump, saying he had “personally good memories and feelings toward Trump”.

She cast doubt on whether her brother had secretly been in communication with Trump, saying: “Perhaps it is the only matter I do not know about in relation to our supreme leadership’s foreign policy.”

She denied claims by Ukraine’s president, Volodymyr Zelenskyy, that North Korea planned to deploy tens of thousands of troops to fight for Russia.