President Barack Obama has ruled out deploying US ground troops in Syria and says military efforts alone cannot solve the country's problems.
"It would be a mistake for the United States, or Great Britain... to send in ground troops and overthrow the [Bashar al-] Assad regime," he told the BBC.
He also said he did not think so-called Islamic State would be defeated in his last nine months of office.
But he said: "We can slowly shrink the environment in which they operate."
Mr Obama, who has been on a three-day visit to the UK, also warned that Britain could take up to 10 years to negotiate trade deals with the US if it votes to leave the EU in a June referendum.