The Syrian conflict entered its bloodiest month in March, with more than 6,000 people killed, including more than 500 women and children, according to the Syrian Observatory for Human Rights.
The UK-based group, which opposes Assad but records human rights abuses on both sides, said around a third of those killed were civilians, in line with previous months.
The death toll included 291 women, 298 children, 1,486 rebel fighters and army defectors as well as 1,464 government troops, reported the Associated Press. The other victims are believed to be unidentified civilians and fighters.
So far the Observatory has recorded 62,554 deaths but believes the real number of those killed to be much higher. The UN says more than 70,000 people have died since the uprising began in March 2011.
Rami Abdelrahman, the head of the group told Reuters by telephone: "We know the number is much, much higher. We estimate it is actually around 120,000 people. Many death tolls are more difficult to document so we are not officially including them yet."
More than 2 million Syrians are believed to have been forcibly displaced within the country since the crisis erupted two years ago and some 4 million have been affected by it.
In July, charity War Child said that young Syrians were being deliberately murdered in execution-style killings, raped, abused, used as human shields and even enlisted against their will to fight.
Attempts to end Syria's two-year-old conflict through peaceful means have failed to make progress.
Last month Foreign Secretary William Hague clashed with EU ministers over whether an arms embargo on Syrian rebels should be lifted.
Hague's Irish counterpart insisted sending more weapons into the war-torn country would result in further casualties, while Hague remained defiant saying he would not rule it out as an option.
A defiant Hague said discussions would centre on reaching agreement across the bloc on the contentious issue.
"That's an option for the UK and France - both countries have said that - but our emphasis is on making sure we keep agreement in the EU," Hague said.
"That's what we have been focusing on so far and we'll have discussions on that."
The UK and France have been vocal in their hopes of arming rebel forces.
The opposition, including the main Syrian National Coalition, says it will accept nothing less than President Bashar Assad's departure from power while Assad's government has vowed to continue the battle until the rebel forces – which it refers to as terrorists – are crushed.