The father of missing 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 press conference in Champaign, Illinois.

Zhang Ronggao was joined by Zhang's mother, Ye Lifeng, and younger brother, Zhang Xinyang, who arrived from China on Saturday.

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

Zhang, a visiting Chinese scholar at the University of Illinois, has been missing since June 9 and is presumed dead by the FBI.

The family has met with the FBI numerous times over the past two months. They said investigators had shared some "good news" about the evidence they've collected, but there have yet to be any breakthroughs on Zhang's whereabouts.

"We totally understand and respect their ways," Zhang's boyfriend Hou Xiaolin said at the conference. "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 can get to the end."

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

Officials of the University of Illinois also announced a yellow ribbon campaign on campus in support of Ms. Zhang and her family, starting Tuesday afternoon. Robin Kaler, associate chancellor for public affairs of the university, said she hopes it will help raise awareness.

Hou said they would continue to raise money and possibly hire a private investigator for the case, which they have not done yet because they did not want to interfere with law enforcement's investigation. The estimated cost would be between $500,000 and $1 million.

"We will never give up on her," Hou said. "I have no energy or time to be anxious, frustrated or angry. 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 the kidnapping of Zhang. He pleaded not guilty and will face trial on Sept 12.

At the end of the conference, a copy of Zhang's diary, put together by the university, was presented to her parents by Chancellor Robert Jones. The diary was found by the police in Ms. Zhang's apartment earlier in the investigation. The original diary is still being held as evidence in the case.

"Life is too short to be ordinary," Zhang wrote on the last page of her diary.