A man (35) sustained bruising and swelling to his face after being assaulted in Nelson.
The incident happened in the early hours of Sunday, when the victim was attacked at a flat above a shop, in Scotland Road.
DC Shaun Hanna, of Burnley CID, said: “We believe that the victim may have tried to gain the attention of passers-by on Scotland Road during the assault and we are looking to speak to anyone who may have been in the vicinity at the time of the attack.
“We are particularly keen to trace two Asian men, aged approximately between 18 and 21 years, who we believe may have spoken to the victim through the window. We believe they may not have realised the man was in distress at the time and we would urge them to come forward with any information.”
Anyone with any information should contact Lancashire Police on 101 quoting log reference LC-20150315-0151.
Alternatively, contact Crimestoppers anonymously on 0800 555 111.