DETECTIVES are urgently appealing for information on a car with taxi markings in connection with Sian O'Callaghan's disappearance.

A green Toyota Avensis estate was seen between Swindon and the Savernake Forest shortly after Sian was last seen on Saturday morning.

Detective Superintendent, Steve Fulcher said: "I am urgently appealing for any witness sightings of a green Toyota Avensis estate with taxi markings seen between 3am and 4am and 12pm to 9pm on Saturday 19th March 2011 between Swindon and Savernake."

Anyone who saw the vehicle or who has information is asked to contact police imediately on 0845 408 7000 or Crimestoppers 0800 555 111, where details may be left annonymously if preferred.

Specialist dog teams are being drafted in to help in the search for the office administrator today after police revealed they were "very close to identifying" her whereabouts.

Search experts have been examining specific "hotspots" within the 4,500-acre Savernake Forest.

Detectives using mobile phone technology pinpointed several areas of interest to the police and allowed them to rule out large areas of the forest.

After a day's intensive examination of the thick undergrowth yesterday, Mr Fulcher said he believed officers were "very close to identifying Sian's whereabouts".

The announcement was made after the detailed search of dense woodland was narrowed following a breakthrough in mobile phone technology.

Around 400 members of the public, including friends and family of the missing office administrator, joined police on Tuesday in the painstaking search of the woodland.

Mr Fulcher said the inquiry was moving at a "rapid pace" with "significant lines of inquiry being developed".

He added: "The public have been fantastic in support of the search for Sian O'Callaghan and I'm very grateful.

"I've been able to use new technological techniques to provide a tighter search parameter and have been able to rule out large areas of the six-and-a-half-mile radius we have all been searching."

Police revealed earlier this week that phone masts put Sian's mobile phone somewhere in the forest just 34 minutes after she left Suju nightclub in Old Town at 2.52am on Saturday.

It was a journey that could only have been made by car, police said.

Sian was caught on CCTV leaving the nightclub after an evening out with girl friends.

The 22-year-old lived in a flat just half a mile away with her boyfriend Kevin Reape, 25, but she never made it home.

On Tuesday, an anonymous donor offered a £20,000 reward for information leading directly to Sian being found.