The Syrian Revolution General Commission said seven people died in fighting in Homs and five people were killed in Hama, The Guardian reported. The organization said five others, including a woman and child, were killed in Idlib.
The Guardian said the Syrian Revolution General Commission's report did not include seven people whose bodies were found in the Damascus suburb of Douma. Earlier, another activist group said eight people were killed in the Damascus suburb.
The fighting between government troops and those opposed to President Bashar Assad's regime came two days ahead of an international gathering meant to organize efforts to end the year-long crisis in Syria, The New York Times reported.
Opposition forces, meanwhile, were trying to position themselves to better influence and coordinate efforts to oust Assad.
In a statement issued Thursday, the Local Coordinating Committees, a coalition of opposition groups inside Syria, complained opponents of Assad weren't represented at an Arab League meeting in Baghdad.
The "Friends of Syria" coalition plans to meet in Istanbul, Turkey, Sunday, to discuss a political transition and the opposition's vision for Syria, officials said.
Syria will "spare no effort" to carry out U.N.-Arab League envoy Kofi Annan's peace plan, but armed groups must end their "terrorist acts," Assad said in a letter to officials of leading emerging economies meeting in New Delhi.
"In return for the formal commitment to the success of Annan's mission, it is necessary to obtain commitments from the other parties to halt the terrorist acts by the armed groups and to withdraw the weapons of these groups and call on them to stop their terrorist acts," said Assad's letter to leaders of Brazil, Russia, India, China and South Africa, which represent nearly half the world's population.
Russia and China are key Syrian allies.
Annan must also persuade countries backing Syria's military opposition to end all types of support immediately, Assad said
Saudi Arabia pressed Jordan March 12 to open its border with Syria to let weapons reach rebels fighting Assad's regime but Jordan has yet to agree, officials from both countries told The Wall Street Journal.
Washington has opposed furnishing arms to the rebels, fearing weapons could end up in the hands of al-Qaida or other extremist groups.
U.S. Joint Chiefs of Staff Chairman Gen. Martin Dempsey said Thursday such a policy could change.
The Syrian opposition appeared to be uniting, a development he said could help clear the way for international aid, including arms, the Journal reported.
Britain said it was doubling non-military aid to Assad opponents and expanding its scope to equipment, possibly including secure telephones to help activists communicate without fear of detection and attack.
The aid, worth $800,000, "includes agreement in principle for practical non-lethal support to them inside Syria," Foreign Secretary William Hague said.
Assad's letter called on "neighboring countries" harboring Syria's military opposition "to stop it immediately" if the Annan peace plan is to succeed, the official Syrian Arab News Agency reported.
Turkey, a former Syrian ally bordering Syria to the north, now hosts the rebel Free Syrian Army, a paramilitary force composed largely of defected Syrian soldiers fighting troops loyal to Assad.
Officials said the Friends group, initiated by French President Nicolas Sarkozy, would seek to find a solution to the Syrian crisis outside the U.N. Security Council, after Russia and China vetoed a resolution on Syria.
Arab leaders at a summit in Baghdad endorsed the Annan peace plan Thursday and called for it to be implemented "immediately and completely."
Annan's six-point peace plan, endorsed by the U.N. Security Council, calls for a Syrian cease-fire, possibly under U.N. monitoring, a withdrawal of heavy weapons and troops from population centers, humanitarian assistance, release of prisoners and free movement and access for journalists.
It does not call for Assad to give up power.
The United Nations says the estimated tally of dead in the 13-month conflict is more than 9,000, up from 8,000 a few weeks ago.