US 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."

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.

The US president later arrived in Hannover, Germany, where he hopes to boost support for the controversial Transatlantic Trade and Investment Partnership (TTIP) deal.

Later, he and German Chancellor Angela Merkel are due to open the Hannover Messe, the world's largest industrial technology trade fair.

No military solution

Obama told the BBC that Syria was a "heart-breaking situation of enormous complexity".

"I don't think there are any simple solutions," he said.

"In order for us to solve the long-term problems in Syria, a military solution alone - and certainly us deploying ground troops - is not going to bring that about."