People are being warned to avoid a mystery substance that's washed up on a number of beaches on Anglesey. Deposits of white and yellow fatty substances have been washed ashore on the northwest coast of the island. The white substance has been found on beaches, including Porth Nobla, Tyn Towyn, Cymyran, Aberffraw and Rhosneigr. A ball of yellow fatty substance has also been found on Cemaes beach. Experts say it could potentially be dangerous to dogs if they try to eat the substance. Anglesey's Maritime Officer, Iwan Huws, has been leading the effort to clear the substance. He explained: "We're received a large number of notifications from the public about these substances. We're grateful for the information provided and have been trying to collect them as quickly as possible. We have also been informed that a number of dogs have become ill after ingesting this substance." He added: "Our beaches remain open, but it advised people to avoid contact with the greasy residue, supervise children carefully, and keep dogs on a lead." Natural Resources Wales is testing the substance, which is believed to be palm or vegetable oil. Sian Williams, from Natural Resources Wales, said: "There are reports about a yellow substance being washed up on Cemaes beach on Anglesey. We are trying to establish what it is, but it could be something like vegetable or paraffin oil. We are working with Isle of Anglesey County Council to tackle this, as well as the white palm oil, which has affected beaches in the area, and would advise people to listen to advice and keep their dogs on a lead." Warning signs are being put up around the beaches.