Barack Obama has warned it is "absolutely critical" that the US pushes to release funds to research a vaccine for the Zika virus.

Health officials are "fairly confident" they can develop an effective vaccine for the disease, the president said - but to do so they require funds which so far have been tied up in Congress.

He called on Congress to end its deadlock on the matter before they retire for the summer recess.

The Zika virus has been linked to defects in newborns such as microcephaly.