The United Kingdom appears to have "worked pretty well", Barack Obama declared as he said America's interest in the Scottish referendum issue was to ensure it retained a "strong, robust, united and effective ally".

The US president was pressed for his thoughts on the question of independence as he appeared alongside Prime Minister David Cameron at a press conference following the G7 summit in Brussels.

"There is a referendum process in place and it is up to the people of Scotland," he told reporters.

"The United Kingdom has been an extraordinary partner to us. From the outside at least, it looks like things have worked pretty well.

"And we obviously have a deep interest in making sure that one of the closest allies we will ever have remains a strong, robust, united and effective partner.

"But ultimately these are decisions that are to be made by the folks there."

Shadow Foreign Secretary and Scottish Labour MP Douglas Alexander welcomed what he said was an "important contribution by President Obama" to the referendum debate.

Mr Alexander said the US President's "clear statement of support for the UK staying together will resonate with many of us here in Scotland".

He added: "As a global statesman President Obama understands that interdependence is a defining feature of our modern world, and that building bridges, not putting up new barriers, is the challenge of our generation."

Scottish Labour leader Johann Lamont said: "For the many Scots who have decided or are even leaning towards a No vote, the endorsement of such a powerful and iconic figure will give them real confidence in their argument.

"President Obama clearly recognises the value of working in partnership and his views on the United Kingdom are spot on. Scotland stands that little bit taller when we work with our neighbours and this is acknowledged by our friends across the Atlantic.

"In his infamous GQ interview, Alex Salmond slagged off President Obama and instead heaped praise on Vladimir Putin. He did not represent the views of many Scots who recognise the achievements of America's first black president and will warmly welcome his views on Scotland."

Scottish Conservative leader Ruth Davidson also welcomed the US President's remarks.

Ms Davidson said: "President Obama has every right to explain to people how the break-up of Britain would affect our relationship with America.

"He clearly recognises the strong and effective partnership his country has with the UK and our many shared achievements through the years.

"Scotland walks taller, shouts louder and stands stronger on the world stage because we are part of the United Kingdom.

"President Obama spoke for many when he said that 'from the outside at least, it looks as if things have worked pretty well'."

She continued: "People in other countries can't understand why you would want to break up one of the strongest, wealthiest and most successful nations on the planet.

"The decision Scotland takes in September will have effects that are felt across the whole of the world.

am proud of the UK's contribution to the international community and understand why our allies and partners want us to stick together."