POLICE put pensioners on alert today after a robber struck for the fourth time in a week and assaulted a 76-year-old woman in her home.

Detectives believe the man is becoming more confident and aggressive following the strikes in the Knuzden area of Blackburn.

They are advising elderly people to keep windows and doors locked and never to let people who call unannounced into their homes.

On Tuesday morning a man knocked on the woman's door in Brantwood Avenue and asked for a drink of water.

Once inside, he grabbed the woman's arm and demanded money. Then he grabbed her face and pushed her backwards.

The woman pushed the man outside and locked it. He fled empty-handed, leaving the woman upset and shaken.

A man with a similar description subjected an 84-year-old woman to what police described as a "terrifying" ordeal at her home in Hozier Street on Saturday.

He entered by an unlocked front door and stole money from the living room before confronting the woman.

She was forced to take him around the house as he threatened violence unless he received more money.

He eventually fled the house leaving the woman distressed and with severe bruising and cuts.

The other two incidents happened last Wednesday afternoon in Didsbury Street.

A man called at the home of a woman in her 70s and said he was from the council to do gardening.

After appearing to work for several minutes, he asked for a drink of water and stole £60 from a purse while she was in the kitchen.

A few hours later a 70-year-old woman walking into her home with her three year-old grandson and pet dog was approached by a man.

She let him use her toilet but he stole a purse with credit cards and cash.

Police are linking all four incidents as the times of attacks, area, method and descriptions are similar.

The man was described as white, aged 25, around 5ft 8ins, with short, dark hair. He was wearing a white vest, and has also been seen with gold chair and blue jeans.

Detective Inspector Neil Hunter said: "We are concerned that this man is gaining in confidence and is becoming increasingly aggressive.

"We know it is difficult, especially in warm weather, to make sure all windows and doors are secure but I would just ask that people bear in mind the opportunist thief who will see an open door as an easy target."

Anyone with information should call Blackburn CID on 01254 353569 or Crimestoppers on 0800 555 111.