Detectives believe three recent shop raids in Oxford could be linked.

Police have not revealed how much cash was taken in each of the raids, but fear the robbers could become more violent.

The first raid was at Threshers, Iffley Road, on May 10 at 9.42pm when thieves jumped over the counter and stole cash.

The second was at Travel Care, London Road, Headington, on May 12, between 4.55pm and 5.05pm. Two men walked in to the travel agents and while one restrained the staff the other forced the branch manager to open the safe. None of the staff were injured.

The third robbery took place in Travel Care, Banbury Road, Summertown, when two robbers burst in and threatened the female staff at 5pm on May 20. One woman was forced in to the cash office

Neither woman was injured.

Similar descriptions of the robbers in each incident have been given to police.

Det Insp Dermot Norridge, of Oxford CID, said: "The level of force used appears to be increasing.

"We want people to cast their minds back to see if they remember anything suspicious.

"We also want shopkeepers to be vigilant and take the necessary security measures to protect their staff, their premises and their takings."

The first man was described as 5ft 11in to 6ft, very dark skinned and of slim build. He was wearing a dark blue zipped-up hooded top with the hood up and pockets at the front, dark blue trousers, possibly jeans, with a light blue patch on the left leg and light blue Nike trainers.

The second man was aged about 19 or 20. He was 5ft 6ins and of skinny build. He has a large, flattish nose. He was wearing a navy blue jumper with the hood up and a blue bobble hat, navy blue trousers with a light blue patch on the left leg and grey and black boxer shorts.

Anyone with information should call Oxford CID on 01865 266333.