The father of missing visiting Chinese scholar Zhang Yingying read a letter the family sent to US President Donald Trump, asking him to "direct all federal resources to try to find Yingying," on Tuesday at a news conference in Champaign, Illinois.

Zhang Ronggao was joined by Zhang's mother, Ye Lifeng, and younger brother, Zhang Xinyang.

The family said they hope their appeal to the president will advance the search for Zhang. They also said they will not give up until Zhang is found.

Zhang, 26, a visiting scholar at the University of Illinois Urbana-Champaign campus, has been missing since June 9 and is presumed dead by the FBI.

The family has met with the FBI many times over the past two months and said investigators had shared some "good news" about the evidence collected, but there have been no breakthroughs on Zhang's whereabouts.

Zhang's boyfriend, Hou Xiaolin, said at the news conference: "We totally understand and respect how the FBI works, but for us it seems that this journey is too long and we don't know where we are. And we don't know when we will get to the end."

Hou said the family would continue using the more than $137,000 raised for their living expenses, a $50,000 reward for information about Zhang's location, legal assistance and other expenses. To date, about $70,000 has been spent.

He said they will continue to raise money and might hire a private investigator, which they had not yet done to avoid interfering in the official investigation. A private investigation had been estimated to cost $500,000 to $1 million.

"We will never give up on her," Hou said. "There's only one thought on my mind - that is to try everything I can to find her."

Brendt Christensen, 28, of Champaign, was arrested on June 30 and charged with kidnapping Zhang. He pleaded not guilty and will go on trial on Sept 12.

At the end of the news conference, university Chancellor Robert Jones presented a copy of Zhang's diary put together by the university, to her parents. The diary had been found by police in Zhang's apartment. The original diary is being held as evidence.

Zhang Ruinan contributed to this story.