The search for a six-year-old boy swept away after falling into a river is to resume, but police have said it is unlikely he will be found alive.

Lucas Dobson slipped and fell into the River Stour in Sandwich, Kent, on Saturday afternoon, while on a fishing trip with family members.

Search teams continued rescue efforts until until nightfall on Sunday and were to resume at first light on Monday morning, Kent fire and rescue service (KFRS) said.

Supt Amanda Tillotson of Kent police said more than 100 members of the emergency services and 200 volunteers had been taking part.

Lucas Dobson. Photograph: Maciee Stanford/PA

She said: “I’ve just come from speaking to the family myself, and they wish to pass on their absolute gratitude for everybody, and they are overwhelmed at the amount of people that have come out to help and search for Lucas.”

Asked how likely a positive outcome was, she said: “Obviously as time has gone on now I think it is unlikely, unfortunately.

“But we will continue to search, and obviously I would like to have a positive outcome. I would like to find Lucas. We are getting increasingly concerned with the amount of time that is going on now.

“But it is really important that we continue to search for Lucas and continue to do that for the family, and my heart goes out to them. It really does.”

She said the search team was working hard “to bring a quick resolution and find Lucas as quickly as we can for the family”.

Tillotson said dive units had searched the river area, along with specialists from other agencies, and that efforts were focused near the site of the incident.

Emergency services were called to Richborough Road at about 1.20pm on Saturday, KFRS said.

Lucas’s aunt, Maciee Stanford, said the boy’s father had jumped into the water after he had fallen between a jetty and a boat, but “the current was too strong. He had already gone”.

She told the MirrorOnline: “He was with his dad and his dad’s friends along with other children.

“The incident happened because our Lucas was on the jetty and tried to step from there on to the boat, but he fell in between the jetty and the boat.

“As soon as he fell the three adults jumped in after [him] but the current was too strong. He had already gone in the short amount of time. He could not be found.

“They fish and do this regularly as it is behind one of the men’s houses. All the children play together here.”

A Facebook page called Search For Lucas said he had been wearing a black and red striped T-shirt with white shorts.

Volunteers were warned about their own safety and told by police to stay away from the water’s edge, work in groups and make sure they had phones if they continued searching through the night.