DAKAR (Reuters) - A vaccination campaign against polio in West Africa will be delayed in some countries because at least 15 million doses are trapped at airports in Germany and France by the ash cloud, a U.N. official said on Tuesday.

The delays have stoked concern that unsynchronised immunisations will allow the virus to spread within the region, Martin Dawes, the regional chief of communications for the U.N. Children's Fund (UNICEF) in West and Central Africa said.

"If there are special flights or if the situation eases, we hope these vaccines could be treated as priority to transport," he told Reuters.

Burkina Faso, Sierra Leone and Togo have postponed by a couple of weeks their participation in the second phase of a campaign to immunise 85 million children under five in 19 countries in West and Central Africa, due to start on April 24.

"This is obviously causing difficulties," Dawes said. "These vaccines have to be kept cool and if this situation lasts longer, they would have to be sent back to manufacturers to ensure that they are preserved."

Ash from an Icelandic volcano has paralysed air traffic in much of northern Europe since late last week, although some restrictions were easing on Tuesday.

Experts say vaccination is the best method of fighting polio, a highly infectious disease caused by a virus that invades the nervous system. The disease, eradicated in most parts of the world, can lead to total paralysis in a matter of hours and is potentially fatal.

To stop the spread of the disease across West and Central Africa, the World Health Organisation, UNICEF and partners began immunising people in the region two or three times a year since 2008. The first phase of this year's scheme began in March.

The vaccination strategy has helped countries such as Benin, Ivory Coast, Central African Republic, Ghana, Togo and Niger contain the disease. The number of cases in Nigeria, which is the only endemic country in the region, dropped to two in 2010 from 48 in the same period last year, according to UNICEF.

However, the quality of the campaigns and their level of coverage have been a problem in Mali, Mauritania, Sierra Leone and Senegal, which has reported 15 cases of polio this year alone, UNICEF said.