Uptake of the MMR vaccine in Wales has still to hit the 95% target
Health officials say the number of suspected measles cases in west Wales has risen from 17 to 20.
The National Public Health Service for Wales (NPHSW) is urging parents to vaccinate their children.
It said there were eight suspected and two confirmed cases in Llanelli, eight suspected cases in south Pembrokeshire, and two more in Burry Port.
It said it did not believe there were any links with north Wales, with 22 suspected cases in Llandudno and Conwy.
However, links have been made between the cases in Llanelli and Pembrokeshire and investigations are continuing to establish whether there are any connections with the cases in Burry Port.
Children aged between one and 16 living in Llanelli and who have not received the full two doses of the measles, mumps and rubella (MMR) vaccination will shortly receive letters inviting them to do so.
Letters have already been sent to children of the same age living in the Pembroke and Pembroke Dock area who require vaccination.
Dr Mac Walapu, consultant in communicable disease control for the NPHS, said: "Measles is a highly contagious infectious disease and children who have not been immunised are at risk.
"Parents can easily protect their children by having them immunised - after completing a two-dose course of MMR, 99% of children will be protected against measles."
Latest figures for measles cases in Wales and England show that there has been an almost 40% increase in the infection compared to 2007.
Dr Walapu added: "Most people make a full recovery but there is a rare possibility of severe complications including serious eye disorders, deafness, brain damage and even death."
GPs are also being alerted to possible increases in demand for the MMR vaccine, said the NHPS.