The Homs Coordination Committee said heavy shelling killed Abdul Rahman Orfalli, 23, one of the principal organizers of the first protests in the city last March, CNN reported.
At least 14 people were killed across the country by mid-morning Tuesday, the majority in Homs, the opposition Syrian Network for Human Rights said.
Most reports from Syria indicate President Bashar Assad's regime is massacring civilians to try to quell the opposition, but some armed opposition members also have carried out abuses, such as kidnapping, detention and torture of security force members, Assad supporters and people identified as belonging to pro-government militias, Human Rights Watch said Tuesday.
"Human Rights Watch has also received reports of executions by armed opposition groups of security force members and civilians," the group said in a statement.
Human Rights Watch has documented widespread human rights violations by Syrian government forces, but "the Syrian government's brutal tactics cannot justify abuses by armed opposition groups," said Sarah Leah Whitson, the organization's Middle East director.
Speaking from Turkey, Sheik Anas Airout of the opposition Syrian National Council said actions presented by HRW "are totally unacceptable," but acknowledged some violations may occur, CNN reported.
"We encourage our free men to show mercy [to] our captives because we want to prove to the world that we are better than the Assad regime, and we will always be," Airout said. "We do not want to repeat the regime's same mistakes."
Meanwhile, the United Nations could vote Tuesday to back U.N.-Arab League special envoy Kofi Annan's mission on Syria.
The U.N. effort is considered a way entice China and Russia -- two countries that have refused to formally condemn the Syrian regime -- to join others nations and organizations in pressuring Syrian leaders to cooperate with Annan, CNN said. He met with Assad earlier this month and presented several proposals to end the crisis.
Both Russia and China, which have trading ties with Syria, have said they want the violence to end, but would not blame the regime alone.
Armed Syrian defectors clashed Monday with security guards in a heavily guarded and wealthy district of Damascus in what activists and residents said was the most intense fighting in the area since the protests began a year ago, The New York Times reported.
The violence in Syria's capital came after a weekend in which bombers hit government targets in Damascus and Aleppo, fueling concerns the armed uprising was moving into places that had escaped much of the violence.
The fighting in Damascus also coincided with the arrival of a monitoring team sent by Annan.
Russian Foreign Minister Sergey Lavrov and International Committee of the Red Cross President Jakob Kellenberger met in Moscow Monday and agreed providing aid to all Syrians was an "absolute priority," the Foreign Ministry said in a statement. The statement said Russia and the Red Cross were urging the Syrian government and opposition forces to "immediately agree to a daily humanitarian pause" so convoys could ferry in medical care and evacuate the wounded.
Russian officials on Monday also denied an ABC News report that a warship docked in the Syrian port of Tartus with anti-terrorism forces on board, the Times said. The Defense Ministry said a tanker with security guards had docked in Tartus 10 days earlier, which may have led to some confusion. He said the tanker supplies fuel to Russian ships that are part of international anti-piracy patrols in the Gulf of Aden.