A convoy believed to be carrying Thailand's fugitive former prime minister Yingluck Shinawatra was last spotted heading through a military checkpoint close to the border with Cambodia, the junta's deputy leader said Friday.

Thailand's first female prime minister, whose government was toppled by the military in 2014, pulled a dramatic disappearing act last month on the day a court was due to deliver a verdict in her trial for criminal negligence.

She has not made any public appearance since her flight but there are widespread reports she has joined her brother Thaksin, who was also toppled in a 2006 coup, in Dubai.

Thailand's junta has said it was unaware she was planning to flee -- something analysts and many Thais have found hard to believe given the round-the-clock surveillance Yingluck frequently complained of.

On Friday, deputy junta leader General Prawit Wongsuwon gave reporters an update on the investigation, saying Yingluck's convoy was last seen on CCTV at a military checkpoint in Sa Kaeo province, which borders Cambodia. 

"The CCTV footage does not show them at the border checkpoint, it finishes at a military checkpoint at Sa Kaeo province," he said, without elaborating on whether soldiers at the checkpoint searched the cars.

It is the first official confirmation authorities have made that Yingluck was last seen heading towards Cambodia.

Junta and officials from Yingluck's Pheu Thai party have given conflicting accounts of the escape route. One senior military official said they believed Yingluck flew straight to Singapore in a private jet and then on to Dubai. 

Party insiders have said she either drove or took a boat to Cambodia from where she then flew in a private plane to Singapore and on to Dubai. 

The government in Phnom Penh has made no public comment on whether she went through their territory.

The junta has come under fire from some conservative allies over Yingluck's disappearance, with many questioning how the authoritarian regime could have let her slip the net.

Any escape through Cambodia would also be embarrassing because the current junta leadership hail from a military clique known as the Eastern Tigers whose power base is in that border region.

Analysts say the military leadership were concerned that jailing Yingluck would afford her martyr status and might reinvigorate her supporters.

The Shinawatra political dynasty began under Thaksin in 2001 with a series of groundbreaking welfare schemes that won them votes and the loyalty of the rural poor.

But their popularity rattled the royalist and army-aligned elite, who assailed successive governments linked to the clan with coups, court cases and protests.

Thaksin himself was toppled in a 2006 coup and fled overseas two years later to avoid jail for a corruption conviction.

The period since then -- dubbed the "Lost Decade" -- has seen frequent deadly street protests, short-lived governments and the return of military rule in 2014.