WASHINGTON (AP/REUTERS) - US President Barack Obama says the United States is working with Britain to keep pressure on the government of Syria, strengthen the moderate opposition there and work to push for the end of the hard-line regime of President Bashar Assad.

He called the violence there appalling and said western allies would work to strengthen the hand of moderate opposition to President Assad's leadership.

Mr Obama spoke at a White House news conference alongside British Prime Minister David Cameron. Their discussions touched on Syria's civil war, the Mideast peace process, a proposed free trade pact with Europe and next month's eight-nation economic summit in Northern Ireland.

Mr Obama also urged Britons to watch whether European Union (EU) reforms were successful before deciding whether to leave the multi-nation bloc, backing the position taken by Mr Cameron.

"I think the UK's participation in the EU is an expression of its influence and its ... role in the world," Mr Obama said.

Mr Cameron is trying to unite his party on the issue after two cabinet ministers suggested they would vote to leave the EU if a referendum were held today.

Mr Obama said the decision was ultimately up to the people of the UK, and noted there were "tough negotiations" ahead on economic and political issues.

"I will say this, that David's basic point that you probably want to see if you can fix what's broken in a very important relationship before you break it off makes some sense to me," Mr Obama said.