By Erika Solomon

BEIRUT, June 18 (Reuters) - Syrian security forces pounded opposition areas across the country on Monday, activists said, adding that at least 23 people had been killed in clashes they say have escalated since international observers suspended their mission.

Activists said artillery had targeted Douma, a town 15 km (9.3 miles) outside the capital Damascus. The town has for weeks been under the partial control of rebels who have joined the 15-month-old revolt against President Bashar al-Assad.

"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."

The British-based Syrian Observatory for Human Rights, which has a network of activists across Syria, said at least 23 people had been killed by midday on Monday, seven of them in Douma.

In a sign it fears Syria's conflict could escalate further, an unnamed 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 and supports Assad's argument that foreign-backed terrorists are behind the unrest. Moscow 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 of nearly two hundred civilians, most of them from the Sunni Muslim majority population 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 international community's efforts to halt the violence are deadlocked because Russia and China, which both wield vetoes in the Security Council, have blocked tougher action against Assad. They say the solution should be through political dialogue, an approach most of the Syrian opposition rejects.

Western powers have been pushing for stronger measures to be taken against Assad, whose forces have not only used artillery in recent weeks, but helicopter gunships against rebels in civilian areas.

U.S. President Barack Obama is expected to discuss the Syria crisis with Russian President Vladimir Putin when they meet in Mexico on Monday. But few observers expect a breakthrough.

Relations between Washington and Moscow have further frayed after a week of Cold War-style recriminations over Syria.

TRAPPED IN BOMBARDED HOMS

The head of the United Nations observation mission, General Robert Mood, is scheduled on Tuesday to brief the U.N. Security Council in New York on the violence in Syria.

The mission recently halted its operations due to security concerns, and Mood said on Sunday he was worried about civilians trapped in central Homs.

"In Homs attempts to extract civilians form the line of fire over the past week have been unsuccessful," he said in a statement. "This requires willingness on both sides (of the conflict) to respect and protect the human life of the Syrian people."

Residents in Homs, the bloody epicentre of the revolt against Assad, said their city has been pummelled daily by mortar and rocket fire since early June.

"It's getting worse since the UN observers suspended their mission," wrote Alaa, who said he was a Homs resident but would only give his first name.

"There are tanks shooting now and most stores are closed. The streets are blocked by security barriers and cement blocks."

Bernard Valero, a spokesman for the French foreign ministry, 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 Kofi Annan's failing peace plan.

France has called on the United Nations to invoke Chapter VII - which can authorise the use of force - to impose the plan brokered by international mediator Annan, including a widely ignored April 12 ceasefire agreement.

U.N. monitors say violence has been escalating rapidly in Syria, where peaceful protests were overtaken by an armed insurgency several months ago in response to Assad's crackdown on dissent.

In Geneva, U.N. High Commissioner for Human Rights Navi Pillay said the government's use of heavy arms and shelling in populated areas could amount to war crimes, and called on the international community to act.

"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," she said. (Additional reporting by Steve Gutterman in Moscow, Tom Miles in Geneva and John Irish in Paris; Writing by Erika Solomon; Editing by Andrew Osborn)