The American Medical Association is urging big social media and tech companies, including Amazon, Facebook and Google, to help combat the spread of misinformation about vaccinations on their platforms.

The nation's largest physicians group sent a letter Wednesday to the chief executives of Amazon, Facebook, Google, Pinterest, Twitter and YouTube, saying it's troubled by reports of anti-vaccine messages and advertisements targeting parents in search of vaccine information. The AMA said this was especially troubling at a time when vaccine-preventable diseases like measles are on the rise in the US, posing a threat to public health.

The AMA said it's concerned that health-related misinformation found online could undermine science and persuade parents to avoid vaccinating their children, sparking the spread of preventable diseases.

"With public health on the line and with social media serving as a leading source of information for the American people, we urge you to do your part to ensure that users have access to scientifically valid information on vaccinations, so they can make informed decisions about their families' health," AMA CEO Dr. James Madara wrote in the letter.

"We also urge you to make public your plans to ensure that users have access to accurate, timely, scientifically sound information on vaccines," he wrote.

The letter comes as companies face mounting pressure from lawmakers, activists and health experts to prevent anti-vax misinformation from going viral. The misinformation may have contributed to an outbreak of measles in the US.

Facebook said last week it's stepping up its efforts to curb the spread of misinformation about vaccines, saying it would demote the ranking of groups or pages that spread misinformation about vaccines on its News Feed and in search results.

Other tech companies have also beefed up their efforts to combat misinformation about vaccines. Pinterest blocked anti-vaccination searches and has tried to pull down anti-vax content. In February, YouTube said it would remove ads from videos that feature anti-vaccination content. 

None of the companies addressed in the letter immediately responded to requests for comment on the AMA's letter.

CNET's Queenie Wong contributed to this report.