By Vassilis Triandafyllou

KOS, Greece (Reuters) - British police began excavations in a field on a Greek island on Monday, saying they had new leads in the disappearance of a British toddler there 25 years ago.

The team began sifting through mounds of earth using a giant sieve. Excavations are expected to continue for days and possibly weeks, with British and Greek forensic archaeologists involved.

South Yorkshire Police, leading the investigation, said they would focus their attention on two sites on the island of Kos close to where Ben Needham was last seen on July 24, 1991.

Detective Inspector Jon Cousins said new information on the case surfaced in May, following a public appeal to Kos residents.

"I have made a decision that there is a very valid and good reason for us to be doing the activity that we are doing here today," Cousins said on the site, an olive grove sealed off by blue-and-white police tape.

"I would not be doing that if I was not optimistic that we are going to find something of significance that hopefully will provide an answer for Ben's family."

He would not comment on British newspaper reports that the child may have been accidentally crushed by a builder's digger.

Asked whether he believed Ben was dead, Cousins replied: "There are many other lines of inquiry in relation to this that are still ongoing.

"I am keeping an open mind, but based on what I know at the moment, based on the assessment of all the information we have, I have made a decision that it is necessary to do the work that we are going to be doing over the next week or so."

Ben was 21 months old when he disappeared while playing outside a farmhouse his family was renovating as a holiday home. A previous dig in the area in 2012 was fruitless.

(Writing by Michele Kambas and Karolina Tagaris; editing by Andrew Roche)