BEIJING, April 19 (Reuters) - China has ordered an investigation into a fire that killed 21 people at a Beijing hospital, one of the deadliest in the capital in recent years, but many social media posts on the incident were deleted as people questioned a delay in announcing the news.

Social media showed dramatic videos of people using tied bed sheets to climb down the walls to escape smoke and flames after the fire broke out around 1 p.m. (0500 GMT) on Tuesday in Changfeng Hospital.

Seventy-one people were evacuated during the rescue. As of 6 p.m., 21 had died, the Beijing Daily reported.

The fire was extinguished in about half an hour, according to local reports.

The Beijing government announced it will hold a press conference at 12 p.m. on Wednesday on the fire. Local media had reported an investigation was in progress.

Authorities blocked access to the hospital on Wednesday. Broken and burned out windows could be seen and there was heavy police presence at the site of the hospital, including plain clothes police, according to Reuters witnesses.

Social media posts about the fire circulating on WeChat for several hours were either censored or deleted, according to checks by Reuters. One post still available criticised the hospital for boasting about its fire preparedness in a February article on its official WeChat account.

"Rescue work at the scene concluded in 3.5 hours, but the public only knew that 21 had died from the fire when it's already past 8 in the evening," one person wrote on WeChat in a post that was later deleted.

"It is very puzzling that little information was known about a fire killing 21 people in a densely populated major city like Beijing before the official notification,” the comment said.

The cause of the fire is being investigated, according to state and local media reports, and there were no further details on injured.