Russian President Vladimir Putin on Sunday insisted that Moscow had abided by "rules and norms" when providing weapons to Syria and demanded other G8 countries which are contemplating arming rebels do likewise.
Putin was speaking in London after holding talks with British Prime Minister David Cameron which could set the tone for the G8 summit, with the West at odds with Moscow over the conflict.
Cameron is seeking to forge an international consensus on handling the unrest as he hosts the leaders of the world's top industrialised nations in Lough Erne, Northern Ireland, from Monday.
Putin, who has provided military support to Syrian President Bashar al-Assad despite pleas from the West, has been dismissive of US claims that the regime has used chemical weapons and insisted Russia had acted appropriately.
"We are not breaching any rules and norms and we call on all our partners to act in the same fashion," he said.
Washington has vowed to send military aid to rebel forces battling to topple Assad after saying it had proof that his regime had crossed a "red line".
Putin warned that countries supplying arms to forces fighting against Assad's regime risked tarnishing their reputation after footage emerged last month of one rebel apparently eating the heart of a dead soldier.
Human Rights Watch and the Syrian opposition National Coalition condemned the video as "horrific".
"It is barely worth it (supplying arms) to support people who not only kill their enemies but open up their bodies and eat their internal organs in front of the public and the cameras," Putin said.
"Do you want to supply these people with arms?
"In that case this hardly has anything to do with the humanitarian values which have for centuries been preached in Europe," he added. "At least in Russia we cannot imagine this."
American officials will not reveal exactly what military support will go to the rebel Supreme Military Council, although by many estimates it will initially be assault rifles and ammunition.
The international community has long been divided over how to tackle the Syria conflict, which according to UN figures has cost at least 93,000 lives since March 2011.
The British prime minister insisted that the talks had convinced him there was still scope for agreement when the world's top leaders meet next week.
"What I take from our conversation today is that we can overcome these differences if we recognise that we share some fundamental aims: to end the conflict, to stop Syria breaking apart, to let the Syrian people decide who governs them and to take the fight to the extremists and defeat them," he said.
Cameron has not said whether he favours sending weapons to the rebels.
Putin had to use the back entrance of Downing Street due to a large protest against the Turkish government blocking the main gates.
The president's plane arrived late at a military airbase in west London and was further held up by the protest.
Putin will meet US President Barack Obama in Northern Ireland on Monday.
The fighting in Syria has taken on increasingly sectarian overtones, pitting a Sunni-led opposition against the Alawite-dominated regime.
US Secretary of State John Kerry, who is pushing for an international peace conference on Syria, warned on Saturday that the chances for a political settlement could be undermined by the regime's use of chemical weapons.
The G8 summit is also likely to consider the impact of the election of moderate cleric Hassan Rowhani as Iran's new president.
Putin urged him to forge closer ties with Moscow, while the United States offered the prospect of direct engagement with Iran but Israel called on world powers to keep up the pressure over the Islamic republic's nuclear drive.
Iran is a key ally of Assad, staunchly backing his embattled government.
The Syrian opposition accuses Iran of providing Damascus with weapons and encouraging Lebanon's Shiite Hezbollah militia, which relies on Tehran for support, to dispatch fighters to Syria.
Besides Syria, Cameron wants the G8 summit to produce new agreements on tax, trade and financial transparency.