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 said.
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 in the UK for a three-day visit, said 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."
Mr Obama said the US-led coalition would continue "to strike ISIL (Islamic State) targets in places like Raqqa, and to try to isolate those portions of the country, and lock down those portions of the country that are sending foreign fighters into Europe".
But he said the international community would have to continue to apply pressure to all parties, including Russia, Iran and moderate opposition groups "to sit down at the table and try to broker a transition".
But he added: "That's difficult."
He criticised those countries whose parliaments had not approved actions in Syria but still "want the United States to do something about it".
"You can't have it both ways," he said.
Mr Obama said Syria was one of many issues that are "transnational in nature... and require a transnational response".
He said: "It would be, I think, tempting, for a lot of people, to believe that we can pull up the drawbridge and that we can carve a moat around ourselves and not have to deal with problems around the world."
But without co-operation and alliances "we are far weaker and we won't solve these problems", he said.
On so-called Islamic State he said: "Prosecuting the campaign is critical, and although I don't anticipate that in the next nine months it will be finished, because, unfortunately, even a small pocket of extremists, if they're prepared to die themselves, can still wreak havoc on many of our cities.
"But I do think that we can slowly shrink the environment in which they operate and take on strongholds like Mosul and Raqqa that are the beating heart of their movement."
At least 250,000 people have died in five years of conflict in Syria and millions have fled the fighting.
Mr Obama and the UN fear a fragile truce between the government forces and non-jihadist rebels, in place since February, is in danger of collapse amid renewed clashes.
Current talks in Geneva between the Syrian government and opposition delegations will continue into next week, but there have been bitter exchanges.
The Syrian opposition delegation, known as the High Negotiations Committee (HNC), and rebel forces inside Syria have accused the government of repeatedly violating the terms of the truce brokered by the US and Russia.
The HNC said the Syrian government was "not a serious partner" while the government hit back by accusing the HNC of "sulking".
Any peace process would require a transitional government, but the role of Mr Assad is a key sticking point.