Pope Francis sent special condolences to those parents who lost their children in a tornado that killed around 100 people in Oklahoma.

"I am close to the families of all who died in the Oklahoma tornado, especially those who lost young children," the Pope said on his Twitter account on May 21.

"Join me in praying for them," he added. Pope Francis also tweeted the same message in Spanish.

Earlier in the day during his morning Mass in the Vatican, the Pope personally added a prayer intention for the tornado victims and those who are missing, especially the children.

There are 20 children among the 91 who have died, but officials said the death toll is expected to increase since the tornado hit southern Oklahoma City suburb of Moore on May 20.

The tornado, which was two miles wide at its greatest, touched down at 2:56 p.m. and lashed the area for 45 minutes with winds of up to 200 mph.

It destroyed homes, businesses, the local hospital and other buildings, including Plaza Towers Elementary School.

Local hospitals have treated at least 145 people in Oklahoma City.

President Obama declared the area a major disaster and will be sending federal aid.

In May 1999, Moore was hit by a tornado that broke records with a wind speed of 302 mph.