LOS CABOS, Mexico/BEIRUT (Reuters) - U.S. President Barack Obama and Russian President Vladimir Putin agreed on Monday that the violence in Syria must stop but gave no sign of agreeing on how to do it even as Syrian security forces pounded opposition areas across the country.
Intense artillery fire was reported in Douma, a town 15 km (9 miles) outside the Syrian capital Damascus that for weeks has been under the partial control of rebels who have joined the 15-month-old revolt against President Bashar al-Assad.
At least 79 people were killed in violence that has escalated since international observers suspended their mission, activists said.
A Russian naval source said Moscow was preparing to send marines to Syria in the event it needed to protect personnel and remove equipment from its naval facility in Syria's Mediterranean port of Tartous, according to the Interfax news agency.
Russia is one of the Syrian government's staunchest backers.
International efforts to halt the violence are deadlocked because Russia and China, which wield vetoes in the U.N. Security Council, have blocked tougher action against Assad. They say the solution must come through political dialogue, an approach most of the Syrian opposition rejects.
Obama and Putin held two hours of talks - longer than originally planned - at a Group of 20 summit in Mexico after a week of Cold War-style recriminations between U.S. and Russian diplomats over Syria. Putin frowned and Obama wore a sober expression during remarks to reporters after the meeting.
"We agreed that we need to see a cessation of the violence, that a political process has to be created to prevent civil war," Obama told reporters.
"From my point of view, we have found many common points on this issue" of Syria, Putin said, adding the two sides would continue discussions.
Obama said they pledged to "work with other international actors," including U.N./Arab League envoy Kofi Annan, to find a resolution.
Obama initiated a handshake for the cameras while the two remained seated. At the end of their statements, as reporters were being ushered out, both sat glumly watching but made no move to re-engage with each other. It was the first Obama-Putin meeting since 2009.
Obama and Western allies want Russia to stop shielding Assad from further Security Council sanctions aimed at forcing him from power. Putin is suspicious of U.S. motives especially after the NATO-assisted ouster of Libyan leader Muammar Gaddafi last year, and has offered little signs of softening his stance.
Though the United States has shown no appetite for a new Libya-style intervention, Russia is reluctant to abandon Syria, a longtime arms customer, and risk losing its last firm foothold in the Middle East, including access to a warm-water navy base.
Russia supports Assad's argument that foreign-backed terrorists are behind the unrest. Russia has repeatedly urged Western and Arab countries, who mostly back the rebels, to rein in their support in order to stem the violence.
International outrage over Syria has grown in recent weeks after two reported massacres in which almost 200 civilians were killed, most of them from the Sunni Muslim majority that has led the revolt. Assad comes from Syria's Alawite minority, an offshoot of Shi'ite Islam that has mostly backed the president.
Heavier fighting and apparent sectarian killings have led many, including the head of U.N. peacekeeping forces, to brand the violence a civil war.
The British-based Syrian Observatory for Human Rights, which has a network of activists across Syria, said 51 civilians and rebel fighters had been killed on Monday, seven of them in Douma. It said 28 members of the security forces were also killed in clashes with rebels in Deir al-Zor, Damascus and Deraa.
"We can't even accurately count the dead because we have so many injured people to treat, there's no time to think about anything else," said an activist in Douma who called himself Ziad.
"The army attacks all the time. They have tanks, missiles, mortars, and artillery. Even helicopters have fired on us. People can't escape because the army is surrounding the town."
Assad's forces have in recent weeks used not only artillery but also helicopter gunships against rebels in civilian areas.
The head of the U.N. observation mission in Syria, General Robert Mood, is to brief the U.N. Security Council in New York on Tuesday, three days after his mission was suspended due to security concerns.
Mood said on Sunday he was worried about civilians trapped in the central city of Homs, epicenter of the revolt against Assad, whose residents say they have been pummeled by mortar and rocket fire almost every day since early June.
French Foreign Ministry spokesman Bernard Valero said the "relentless repression of the regime, and in particular in the city of Homs" meant it was more necessary than ever for the United Nations to enforce Annan's failing peace plan.
France has called on the United Nations to invoke Chapter VII, which can authorize the use of force, to enforce the plan, under which the Syrian army was to withdraw heavy weapons from towns and cities and both sides were to cease fighting in April.
In Geneva, U.N. High Commissioner for Human Rights Navi Pillay said the government's use of heavy weapons in populated areas could amount to war crimes, saying: "I urge the international community to overcome its divisions and work to end the violence and human rights violations to which the people of Syria have been subjected."
Additional reporting by Gleb Bryanski in Los Cabos, Mexico, Steve Gutterman in Moscow, Tom Miles in Geneva, Dominic Evans in Beirut and John Irish in Paris; Editing by Will Dunham