David Cameron will support US plans to impose a no-fly zone over parts of Syria, as he attempts to convince world leaders to act against the “dictatorial and brutal leader” President Bashar al-Assad during the G8 summit in Northern Ireland this week.

The Prime Minister is expected to discuss the dramatic escalation of international involvement in the Syrian civil war at a meeting with Mr Assad’s ally, Vladimir Putin, in Downing Street on Sunday.

He will press the Russian President to sanction a catalogue of further measures against the Syrian regime, leading up to a possible no-fly zone, marshalled by US and allied jets and Patriot missiles operating from across the border in Jordan.

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The push for action comes after Barack Obama said he would give “direct military aid” to Syrian rebels, citing evidence that Mr Assad’s forces had used chemical weapons.

Two years ago, the Prime Minister committed RAF aircraft to policing a no-fly zone over Libya, arguing that “if Colonel Gaddafi uses military force against his own people, the world cannot stand by”. Mr Cameron will insist that President Assad “is now guilty of the most appalling crimes against his people”, in an interview to be televised today.

The US made it clear that it would join with Britain and France to urge President Putin to drop his political and military support for the Syrian leader. “It’s in Russia’s interest to join us in applying pressure on Bashar al-Assad to come to the table in a way that relinquishes his power and his standing in Syria,” said Ben Rhodes, Mr Obama’s deputy national security adviser. “We don’t see any scenario where he restores his legitimacy to lead the country.”

Whitehall sources said yesterday the Government hoped the summit would produce a “route map” towards a solution to the crisis, with staging posts including economic sanctions, a no-fly zone – already discussed at a number of international summits – and direct military intervention. All of these would require United Nations – and Russian – support, so the arming of Syrian rebels is seen as a more direct first step.

But Russia underlined its determination to prevent an escalation of the conflict after Foreign Minister Sergei Lavrov questioned claims that Syria had used chemical weapons – and warned that a no-fly zone would “violate international law”.

Mr Cameron has pledged that the Syria crisis will be “up front and centre” when leaders of the world’s most powerful economies meet for the G8 summit of world leaders in Enniskillen this week. As he prepares his diplomatic offensive, however, a poll for The Independent on Sunday today shows that only a third of the British public thinks Mr Cameron has shown strong leadership in his dealings with other countries.

The Prime Minister also faced growing unrest from opponents and within his own party over the wisdom of committing Britain to any military involvement in Syria. Senior Liberal Democrats warned that a move to involve the armed forces in another conflict would effectively destroy the coalition. Deputy Prime Minister Nick Clegg has repeatedly stressed that any initiatives must be aimed at producing a political solution to the crisis. The Lib Dem leader said the Government would use the G8 “to work out with international partners how to respond to the evidence of the use of chemical weapons”.

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But his predecessor, Sir Menzies Campbell, told The IoS: “The idea that a no-fly zone constitutes non-intervention is difficult to justify. You may not have boots on the ground but you would most certainly have wings in the air. The political reality here is that, so far, there is no sign of a majority in the House of Commons for the supply of arms in any form to the rebels. A decision of this kind would not only be difficult in Parliament, but it would undoubtedly strain the coalition.”

Tory MP John Baron, a member of the Foreign Affairs Committee, said: “We must guard against ‘mission creep’. The more we edge closer to direct involvement, the more we become responsible for events on the ground. And the more we would find it difficult to extricate ourselves.”

Mr Cameron was central to the development of the international no-fly zone over Libya in 2011, which resulted in thousands of sorties and strikes against military targets on the ground. Ministers have previously judged that a similar operation over Syria was not so urgent, as Mr Assad’s air force has been less active than Gaddafi’s, but they maintain that “all options remain on the table”.

US sources have in recent days confirmed that a no-fly zone is under discussion. The US has already moved Patriot missiles and fighter jets into Jordan as part of an annual exercise, but made clear that the military assets could stay on.

Mr Cameron backed the lifting of an EU arms embargo on Syria in May, but says he has made no decision to send arms to the Syrian opposition. In an interview with Dermot Murnaghan on Sky News today, he will stress the need to work with the Syrian opposition, “because, after all, they are trying to defend their communities against appalling attacks, including, let’s be clear, chemical weapon attacks”.

The rhetoric of war

“The participation of Shia militias in Syria under the pretext of protecting holy shrines is a sectarian project endangering the whole region.”

Hamid al-Mutlaq, Iraqi opposition Sunni lawmaker

“The Syrian mayhem is turning into a sectarian conflict between Sunnis and Shias that is ripe to explode and impact the whole region. This war will bring forward centuries-old grievances.”

Abdullah al-Shayji, Kuwait University

“The danger of sectarian violence in Syria is bound to destabilise the entire region.”

Ekmeleddin Ihsanoglu, secretary-general, Organisation of Islamic Cooperation (OIC)

“Syria has become the battlefield for an international war.”

Abulhassan Banisadr, former president of Iran

“This is no longer a civil war between fractions within Syria. We should be taking a more interventionist line.”

Tony Blair, former British prime minister