Robbers armed with knives and martial arts weapons have carried out a series of three robberies in Oxford.

Police are appealing for witnesses after the robberies in Headington but would not comment today on whether they thought the crimes were committed by the same gang.

The first robbery happened on Friday, September 26, at about 10pm when a man walking along Roosevelt Drive away from Warneford Halls of Residence was approached from behind by two men.

One brandished a knife and demanded the man hand over his MP3 player — he refused but handed over £15 in cash before the thieves ran off towards Gypsy Lane.

Police described one man as aged between 18 and 19. He was between 5ft 7in and 5ft 8in, with pale, clear skin, and dark, fairly short hair.

He was wearing a hooded top with the hood up and had his mouth covered.

The knife he threatened the man with had a green and red striped handle and a blade of about 10in.

The other man was carrying holding a pair of martial arts nunchucks - two sticks linked by a chain - and was aged about 19. He has pale skin with spots.

He was described as skinny and about 5ft 6in, with dark hair. He was wearing a grey or white hooded top with the hood up and had his mouth covered.

At 10.30pm on Saturday, September 27, two men approached two women who were sitting on a bench outside the halls of residence.

They threatened the women with knives as they asked for their money. One of the women handed over £30.

One of the robbers was described as between 18 and 20, about 5ft 10in to 6ft and of medium build.

He was wearing a black hooded top with the hood up and had a black scarf, dark trousers and dark boots.

The other man was about 18 to 20, about 5ft 7in or 5ft 8in, and was wearing a grey hooded top, dark boots and dark trousers. He had a knife with a three-inch blade.

On Friday, at about 8pm, a man was walking in the park between Cheney Lane and Gypsy Lane when two men came up to him, threatened him with a knife and demanded he hand over his iPod.

The man with the knife was described as white, aged about 16, 5ft 10in and skinny.

He was wearing grey tracksuit bottoms, a black hooded top and chunky shoes.

The other man is thought to be black, but he had a hooded top pulled over his face. He was also wearing black jogging bottoms and a peaked baseball cap and was carrying a pair of nunchucks.

Anyone with information should call Oxford police on 08458 505505 or Crimestoppers on 0800 555111.