UK PM David Cameron met Russian leader Vladimir Putin - Syria's key ally - on Sunday. They will each hold separate talks with President Barack Obama, who has indicated he will arm the rebels.

Mr Cameron, the host, is also keen to focus on global economic issues.

He hopes to oversee the launch of talks for an EU-US free trade deal and achieve progress on tax transparency.

The 39th Summit of the Group Of Eight (G8) will be held in Lough Erne, County Fermanagh, on Monday and Tuesday.

Canada, France, Germany, Italy, Japan, Russia, the US and the UK will be represented.

Mr Cameron said he wanted "a meeting where we can look each other in the eye, cut through the obstacles and the opposition and generate the political will to solve the problems we face".

The prime minister is scheduled to meet Mr Obama ahead of the opening of the summit.

But it comes amid allegations, made in The Guardian newspaper on Monday, that Britain spied on delegates who attended two G20 meetings in London in 2009.

The newspaper said documents, leaked by the ex-CIA whistleblower Edward Snowden, showed that British intelligence monitored the computers of foreign politicians and officials.

British spies are accused of setting up internet cafes to read delegates' email traffic and penetrating the security on officials' BlackBerrys to monitor email messages and phone calls. Targets are alleged to have included the Turkish finance minister and possibly 15 others in his party.

The newspaper says the documents suggest the operation was sanctioned in principle at a senior level in the government of the then Prime Minister Gordon Brown.

Downing Street has not yet commented on the revelations, which could cause tensions among delegates attending this week's G8.

After his meeting with Mr Putin, the UK PM said it was no secret he and the Russian president had disagreements over Syria, but that they shared a common aim - to end the conflict.

Mr Putin said "blood is on the hands" of both the Syrian government and the rebels in the crisis and that Russia was not breaching any laws by supplying arms to the "legitimate government of Syria".

On Friday, the US announced it would supply some rebels with direct military aid after seeing evidence of chemical weapons use by Bashar al-Assad's regime.

Mr Cameron backed the lifting of EU arms sanctions against the rebels, but says no decision has been made on whether the UK will provide arms to them.

Two years of civil war in Syria has left an estimated 93,000 people dead.

Mr Putin will meet separately with Mr Obama on Monday evening.

In addition to Syria, they may discuss nuclear arms reduction and counter-terrorism in the wake of the Boston Marathon bombings.

One other major foreign affairs issue to be discussed at the summit will be the election at the weekend of Hassan Rouhani as the new Iranian president.

Before the summit opens, Mr Cameron and Mr Obama will meet Italian PM Enrico Letta, German Chancellor Angela Merkel and French President Francois Hollande to signal the start of talks on the EU-US free trade deal.

Mr Cameron has said the deal could be worth £10bn ($15.7bn; 11.8bn euros) to the UK, adding: "That's not some abstract statistic, these trade deals matter, because they mean more jobs, more choice for consumers and lower prices."

The formal talks on Monday are scheduled to cover the global economy.

On Tuesday, Mr Cameron will hope to make progress on tax transparency after agreeing a deal on the issue over the weekend with British overseas territories and Crown dependencies.

He has made no secret of his desire to tackle tax evasion and aggressive tax avoidance.

Tuesday will also cover counter-terrorism issues.

Mr Obama, on his first visit to Northern Ireland, is scheduled to deliver a public address on Monday morning at the Waterfront Hall centre in Belfast.

A White House spokesman said: "He will deliver remarks and engage with the people of Northern Ireland and highlight the hard work, dialogue and institutional development they have undertaken together to advance peace and prosperity."

Security in Northern Ireland is tight, as ever for the G8, which has been a magnet for protests in the past.

A four-mile (6.5km) long, 3m-high metal fence surrounds the golf resort where the summit takes place.

Some 8,000 police officers will be deployed for the summit.

Police are expecting an anti-G8 march in Enniskillen in County Fermanagh on Monday, with about 2,000 demonstrators.

Alistair Finlay, Assistant Chief Constable with the Police Service of Northern Ireland, said: "We are ready for all the eventualities that we may be faced with. Our indications are now that this is hopefully going to be a very peaceful time."