​U.S. President Donald Trump has reportedly signed an executive order that prioritizes Americans' access to COVID-19 vaccines, in a move that could prevent international vaccine access until Americans have received the shots. 

According to foreign media on Tuesday, Trump signed the executive order at the White House, highlighting the speedy development of vaccine candidates and their expected authorization.

Right after signing the order, Trump reportedly said that if necessary, he would invoke the Defense Production Act to make sure Americans would get the vaccines first. 

The executive order says the government must ensure Americans have access to the vaccine before it shipped to other countries. 

However, AFP said that it's unclear how the order would be enforced given that drug manufacturers are obliged to honor contracts signed with other governments.