A purported ransom deadline for the safe return of Savannah Guthrie's mother has passed, shortly after the US news anchor issued a fresh plea for help.

In a video posted on Instagram, Guthrie said her family is "at an hour of desperation".

As the Monday deadline of 17:00 local time (00:00 GMT) approached, officials said they had still not identified a suspect or person of interest in the investigation into Nancy Guthrie's disappearance.

Nancy Guthrie disappeared in the middle of night from her home in Tucson, Arizona, and was last seen on 31 January. Authorities believe she was taken against her will.

The FBI said they are not aware of "any continued communication between the Guthrie family and suspected kidnappers". It is offering a $50,000 (£36,000) reward for any information.

The FBI also said it is taking an email with a ransom note sent to US media outlets stating the Monday deadline seriously, the BBC's US partner CBS reported.

A previous alleged deadline for ransom, which was meant to be paid in Bitcoin, passed on 5 February.

Nancy Guthrie was reported missing when she did not attend a Sunday church service on 1 February and other members grew concerned.

The family has released three previous videos saying they were willing to speak to the potential kidnapper, and asking to provide proof that her mother was alive.

The family had earlier indicated that they would be willing to pay for her safe return.

"I just want to share a few thoughts as we enter another week of this nightmare," Guthrie says, appearing alone in the latest video, in contrast to previous recordings where she has appeared with her siblings.

"She was taken, and we don't know where. And we need your help," Guthrie says.

Guthrie requests people everywhere, "even if you're far from Tucson", to report anything suspicious that could lead to clues about her mother.

She continues: "If you see anything, if you hear anything, if there's anything at all that seems strange to you, that you report to law enforcement".

In an update just before Monday's deadline, the Pima County Sheriff's Office said it did not have any new information to release publicly.

The sheriff's statement added that the "investigation into Nancy Guthrie's disappearance remains active and ongoing", and that "the FBI is handling all aspects related to any ransom note(s) or communications involving the Guthrie family".

The FBI also released a statement, saying that FBI agents from around the country "continue to deploy to Tucson".

"We are currently operating a 24-hour command post that includes crisis management experts, analytic support, and investigative teams. But we still need the public's help," the FBI statement added, including the number for the FBI's tip line.

At a news briefing last week, FBI Special Agent Heith Janke said his agency was reviewing a ransom note sent to several media outlets.

The note gave a deadline for payment of 17:00 on Thursday, he said, without specifying which time zone, and had a second deadline for Monday.

On Monday, FBI Director Kash Patel arrived in Tucson, on a trip that was scheduled prior to Guthrie's disappearance. (BBC)