AMMAN/LONDON – Russian President Vladimir Putin, arriving in Britain before an international summit set to be dominated by disagreement over the U.S. decision to send weapons to Syria's rebels, said the West must not arm fighters who eat human flesh.

In Syria, rebels fought back on Sunday against forces of President Bashar al-Assad and his Lebanese Hezbollah allies near Aleppo, where Assad has announced a campaign to recapture the rebel-held north after seizing a strategic town this month.

A huge explosion shook the Mezze military airport on the western edge of Damascus, a major base for Assad's elite Republican Guards and a source of rocket and artillery attacks against rebellious Sunni Muslim neighborhoods.

The Syrian Observatory for Human Rights, a monitoring group based in Britain with a network of observers in Syria, said it appeared that a booby-trapped car had exploded at a roadblock near the compound, killing or wounding around 20 members of Assad's forces.

After months of deliberations, Washington decided last week to send weapons to the rebels, declaring that Assad's forces had crossed a "red line" by using nerve gas.

The move throws the superpower's weight behind the revolt and signals a potential turning point in global involvement in a two-year-old war that has already killed at least 93,000 people.

It has also infuriated Russia, Cold War-era ally of Syria, which has sold arms to Assad and used its veto at the U.N. Security Council to block resolutions against him.

Russia has dismissed the U.S. evidence that Assad's forces used nerve gas. The White House said President Barack Obama will try to lobby Putin to drop his support for Assad during this week's G8 summit hosted by British Prime Minister David Cameron.

After meeting Cameron in London, Putin said Russia wanted to create the conditions for a resolution of the conflict.

"One does not really need to support the people who not only kill their enemies, but open up their bodies, eat their intestines in front of the public and cameras," Putin said.

"Are these the people you want to support? Are they the ones you want to supply with weapons? Then this probably has little relation to the humanitarian values preached in Europe for hundreds of years."

The incident Putin referred to was most likely that of a rebel commander filmed last month cutting into the torso of a dead soldier and biting into a piece of one of his organs.

Both sides have been accused of atrocities in the conflict. The United States and other countries that aid the rebels say one of the reasons for doing so is to support mainstream opposition groups and reduce the influence of extremists.

Doubts over conference

The U.S. plan to arm the rebels also places new doubt over plans for an international peace conference called by Washington and Moscow, their first joint attempt in a year to try to seek a settlement.

After meeting Putin, Cameron said the divide between Russia and the West over Syria could be bridged, although they disagreed about who was at fault.

"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."

Britain has not said whether it too will supply arms to the rebels, but the issue is contentious even within Cameron's Conservative-led government. Nick Clegg, the deputy prime minister from the Liberal Democrat coalition partners, said: "We clearly don't think it's the right thing to do now, or else we would have done it."

Under its new posture, Washington has also said it would keep warplanes and Patriot surface-to-air missiles in Jordan, an ally whose territory it can use to help arm and train rebel fighters. Washington has 4,500 troops in Jordan carrying out exercises.

Washington has not ruled out imposing a no-fly zone over parts of Syria, perhaps near the Jordanian border, although it has taken no decision yet to do so.

Jordan's King Abdullah rallied his own armed forces on Sunday, telling military cadets: "If the world does not help as it should, and if the matter becomes a danger to our country, we are able at any moment to take the measures to protect the country and the interests of our people."

Washington hopes its backing will restore rebel momentum after Assad's forces seized the initiative by gaining the open support of Hezbollah, Lebanon's Iranian-backed Shi'ite militia, which sent thousands of seasoned fighters to aid Assad.

Just a few months ago, Western countries believed Assad's days were numbered. But with Hezbollah's support he was able to achieve a major victory this month in Qusair, a strategically located rebel-held town on a main route from Lebanon.

Fight for Aleppo

Since then, the government has announced major plans to seize the north, including Aleppo, Syria's biggest city and commercial centre, largely rebel-held for nearly a year. The United Nations said it fears for a bloodbath in the north.

Rebels say they are fighting back against government offensives in the north. An opposition operations room in northern Aleppo said fighters had destroyed an army tank and killed 20 troops at Marat al-Arteek, a town where opposition sources say rebels are holding back an armoured column sent to reinforce loyalists from isolated Shi'ite villages.

"Assad's forces and Hezbollah are trying to control northern rural Aleppo, but they are being repelled and dealt heavy losses," Colonel Abdeljabbar al-Okeidi, a Free Syrian Army commander in Aleppo, told al-Arabiya Television.

He said Hezbollah had sent up to 2,000 fighters to Aleppo and the surrounding areas, but expressed confidence that the opposition would prevail.

"Aleppo and Qusair are different. In Qusair we were surrounded by villages that had been occupied by Hezbollah and by loyalist areas. We did not even have a place to take our wounded. In Aleppo, we have a strategic depth and logistical support and we are better organised," he said. "Aleppo will turn into the grave of these Hezbollah devils."

Battles were also fought inside Aleppo itself, where thousands of loyalist troops and militiamen reinforced by Hezbollah have been massing and attacking opposition-held parts of the city, driving rebel fighters back.

Opposition activists said the army was also airlifting troops behind rebel lines to Ifrin, in a Kurdish area, which would give access for a bigger sweep inside the city.

"For a week, the rebel forces have been generally on the retreat in Aleppo, but the tide has started turning in the last two days," said Abu Abdallah, an activist in the area.

Hezbollah's support for Assad, a follower of the minority Alawite offshoot of Shi'ite Islam, against mainly Sunni Muslim rebels has increased fears of sectarian violence spreading into neighbouring countries.

In Lebanon, security sources said gunmen had shot dead four Shi'ite Muslim men in an ambush in the Bekaa Valley close to the Syrian frontier. It was not clear who was behind the shooting.

Lebanon is still rebuilding from its own sectarian civil war, fought from 1975-1990. Fighting between Sunnis and Shi'ites was also behind most of the violence in Iraq in the decade after the U.S. invasion of 2003.