Syrian troops bombarded a rebellious suburb of the nation's capital with tank and artillery shells Friday, killing dozens of people during a particularly bloody few days across the country, activists said.

The violence is part of a fierce government offensive aimed at regaining control of parts of Damascus suburbs where rebels operate. Major world powers were to meet Saturday in Geneva to hash out a political transition plan for Syria, which has been convulsed by more than 15 months of violence.

It's difficult to get an accurate death toll in tightly controlled Syria, where journalists and human rights groups are either banned or severely restricted. But two opposition groups that compile and document casualties reported the death of more than 125 civilians in fighting across the country on Thursday alone.

Death tolls often take several days to compile because of the restrictions and chaos in the country.

The British-based Syrian Observatory for Human Rights said Thursday's toll included more than 60 soldiers. If confirmed, it would be one of the highest death tolls on a single day since the start of the uprising against President Bashar al-Assad in March 2011.

Activists said at least 43 people were killed in more than two days of shelling in the sprawling Damascus suburb of Douma, which has been a hotbed of dissent and has put up strong resistance to the Assad regime. The dead included three children and five members of a single family.

A local activist who spoke on condition of anonymity for security reasons said the shelling was "relentless" throughout Thursday, and exploding shells killed people in their homes.

"They (government troops) are trying to bring Douma under control, but they are being met by fierce resistance," said Rami Abdul-Rahman, the group's director. He said most of the dead were civilians.

Most victims killed in suburb of Douma

The Local Co-ordination Committees network said 59 people were killed in Thursday's shelling of Damascus suburbs, most of them in Douma. The difference in tolls could not be reconciled.

The state-run SANA news agency said troops continue to pursue "terrorist groups" in Douma, raiding their hideouts and destroying their communications and other equipment. Clashes resulted in the death of dozens of terrorists — the official term authorities use for rebels — and the wounding and arrest of many others, the statement said.

An image from amateur video released on May 20, 2012, purports to show a Syrian government solider aiming his gun in the Damascus suburb of Douma. (Associated Press)

Amateur videos posted by activists online showed bloodied bodies lying on blankets in a room and others shrouded in white sheets and placed on stretchers. "A new massacre by Bashar Assad," cried a man holding a dead girl in a pink blouse, a large gash on her face.

The violence around the capital's suburbs mirrored fighting across many parts of Syria that killed dozens of other people Thursday, according to the activist groups.

They say more than 14,000 people have been killed since the uprising began in March 2011.

Much of the violence that has gripped Syria has been sanctioned by the government to crush dissent. But rebel fighters are launching increasingly deadly attacks on regime targets, and several huge suicide bombings this year suggest al-Qaeda or other extremists are joining the battle. A bomb blast rocked central Damascus on Thursday near a busy market and the country's main justice complex, wounding at least three people and sending a cloud of black smoke into the air.

The latest carnage came as world powers show new urgency to resolve the crisis, which so far has resisted international efforts.

Transition plan facing hurdles

On the eve of Saturday's conference in Geneva, U.S. Secretary of State Hillary Rodham Clinton and Russian Foreign Minister Sergey Lavrov were to meet in St. Petersburg in a bid to iron out deep differences over the transition plan being pushed by UN envoy Kofi Annan.

Annan's plan calls for the formation of a national unity government that would oversee the drafting of a new constitution and elections.

U.S. officials are adamant that the plan will not allow Assad to remain in power at the top of the transitional government, but Russia insists that outsiders cannot dictate the ultimate solution or the composition of the interim administration.

Annan laid out his expectations for the weekend conference in an op-ed in The Washington Post. The future government in Syria, he said, "must include a government of national unity that would exercise full executive powers. This government could include members of the present government and the opposition and other groups, but those whose continued presence and participation would undermine the credibility of the transition and jeopardize stability and reconciliation would be excluded."

Such a proposal does not explicitly bar Assad, but the U.S. and other Western powers who will participate in the conference said that is implicit.

Russia is Syria's most important ally, protector and supplier of arms. Diplomatic hopes have rested on persuading Russia to agree to a plan that would end the Assad family dynasty, which has ruled Syria for more than four decades.

There are few options besides keeping up diplomatic pressure, as an international military intervention is all but ruled out in the near future. Few countries are willing to get deeply involved in such an explosive conflict, and Russia and China have pledged to veto any international attempt to intervene militarily.