Rami Abdul-Rahman, who heads the Syrian Observatory for Human Rights, said the increased toll is likely incomplete because both the Syrian army and the rebel groups fighting President Bashar Assad's government often underreport their dead in the civil war.
"Both sides are hiding information," Abdul-Rahman said by phone from Britain, where his group is based. "It is very difficult to get correct info on the fighters because they don't want the information to hurt morale."
The numbers, while provided by only one group, support the appraisal of the conflict offered by many Syria watchers: The civil war is largely a military stalemate that is destroying the country's social fabric and taking a huge toll on civilians.
The increase also reflects the continuing spread of major hostilities to new parts of Syria. While clashes continue in Aleppo, Damascus and Homs, Syria's three largest cities, rebels have launched an offensive in recent weeks to seize towns and army bases in the southern province of Daraa, largely with the help of an influx of foreign-funded weapons.
The Observatory, which works through a network of contacts in Syria, said those killed in March included similar numbers of combatants on both sides: 1,486 rebels and army defectors and 1,464 soldiers from the Syrian army.
But the number of civilians killed exceeded them both: 2,080 total for the month, including 298 children and 291 women.
In addition, there were 387 unidentified civilians and 588 unidentified fighters, most of them foreigners fighting with the rebels, bringing the March total to 6,005, Abdul-Rahman said.
He criticized the international community for not doing more to stop the bloodshed, which he said could increase.
"If there is no solution, we think the numbers will get worse in the coming months," he said.
The March toll surpassed what had previously been the deadliest month, August 2012, when airstrikes, clashes and shelling killed more than 5,400 people, Abdul-Rahman said.
His total death toll for the conflict through the end of March was 62,554, a number he acknowledged as incomplete, suggesting the true figure could be twice as high.
Besides the under-reporting of dead fighters by both sides, he mentioned the tens of thousands of missing persons and captives held by the regime and the rebels. The fate of these people is rarely uncovered, he said.
He also said more than 12,000 pro-government gunmen known as "shabiha," along with government informers may have been killed by the opposition and never reported.
The constant stream of new reports, in addition to the lack of free access to much of the country, makes full investigations impossible.
"Since there are more dying every day, it is very hard to go back and document those who died before," Abdul-Rahman said, calling for an independent international investigation inside Syria.
The Observatory's numbers are not as high as those given by the United Nations.
On Feb. 18, a UN-appointed Commission of Inquiry on Syria issued a 131-page report saying about 70,000 people had been killed in the conflict. The report compiled and corroborated death reports from a number of different sources.
The UN has not updated its number since.
The Syrian government does not provide regular death tolls for the conflict. Syrian officials did not immediately comment on the reported death toll.
Assad's regime describes the conflict as a foreign conspiracy to weaken the country carried out by terrorists on the ground.
In an attempt to boost that argument and rally regime supporters, Assad's wife, Asma, broke her long silence on the events shaking the country in a video shown on Syrian TV stations over the weekend and posted on the Internet.
In the professionally produced 14-minute video, she was seen greeting, hugging and kissing women who were described as the mothers of Syrian soldiers killed in battle.
The video, titled "With Your Soul, Protect the Jasmine," said it was filmed during a reception on Mother's Day, which is celebrated in much of the Arab world on March 21. Asma Assad, dressed casually and speaking in Arabic, thanked the mothers for their sacrifice.
"Instead of fearing for yourselves, fearing for your lives, you feared for all of Syria," she said. "Instead of your children fearing only for you, they feared for all the mothers in the country. They went to protect the country knowing that Syria, the homeland, is the mother of all."
When she finished her speech, a girls' choir broke into a patriotic song.
The video, which was posted on Friday on the official Facebook page of the president's office, is the first time Asma has spoken out in public since the start of the conflict. Her silence had prompted some to speculate that the British-born first lady disapproved of the regime's violent crackdown on the opposition.
She appeared briefly at a pro-regime rally in January 2012, smiling with her children as her husband said the "conspiracy" against Syria was in its final stage.
A month later, she accompanied her husband to a polling station during a referendum on a new constitution, but did not speak.
In recent weeks, the president's office has published photos of her visiting the children of people killed in the civil war.