Searches are continuing in Papa Westray, Orkney, to trace Sergei Ivanov and officers would urge anyone who thinks they may have information which could assist police to get in touch.
Sergei, 55, was last seen at around 10am on Monday, 3 April walking on a road near to his home address towards the South of the island.
He was reported missing on Sunday, 9 April.
Inspector Keith Bendall said: "Coastline and land searches have been carried out and police have been assisted by the Coastguard including the use of the helicopter.
"As time goes by we are increasingly worried for Sergei's welfare and it is out of character for him not to be in touch with family and friends.
"We would urge anyone who may have information to contact police as a matter of urgency on 101.
"The community has supported police and we would like to thank them for that. In addition we would ask people to check any sheds or outbuildings they have for signs that someone may have sought shelter there. The forecast is poor for today with high winds however we will continue searches where possible."
When Sergei was last seen he was wearing a black jumper, black trousers, long black coat (below his knees) and black work type boots.
Sergei is described as around 5ft 10ins tall, with long grey hair and a medium build.
Call 101 for non-emergencies and general enquiries, in an emergency call 999. If you have information about a crime you can also contact Crimestoppers anonymously on 0800 555 111.