The Syrian opposition said that it halted a measles vaccination campaign in the northeastern Idlib province following reports children had died after being innoculated.
The announcement was posted on the websites of the Syrian opposition National Coalition and the rebel interim government.
The "interim government's health ministry has instructed a halt to the second round of the measles vaccination campaign, which began Monday... following several fatalities and injuries among children in vaccination centres in the Idlib countryside," the statement said.
It stressed that a first round of vaccinations against measles, which began a month ago, had been carried out "without any problems."
They said the vaccines being used in the latest campaign had come from the same source as those in the previous round, without specifying what that was.
The Syrian Observatory for Human Rights, a Britain-based NGO, said "at least 12 children have died and dozens more are suffering from poisoning or allergic reactions after measles vaccinations."
The death toll was revised from an earlier figure of five, and the group said it could rise further because several children were in serious condition.
The group said medical sources had suggested that the vaccines could have been compromised, either because they were expired or poorly stored, but there was no confirmation.
Medical groups have rushed to head off the spread of measles, mumps, rubella and polio in Syria, where normal medical services have disintegrated because of the civil war, which erupted in 2011.
The United Nations said earlier this year that 1.6 million children were to be vaccinated in Syria against polio, measles, mumps and rubella.