Syrian troops pursued armed defectors near the border with Turkey, sparking clashes in which 12 people died, and crossed into Lebanon to gun down a Syrian national, activists and an official said.
The surge of violence comes as the European Union prepared fresh sanctions against President Bashar al-Assad's regime following the failure of a UN resolution calling for "targeted measures" if he pursued his brutal clampdown on dissent.
The United Nations said it now estimated the crackdown had killed more than 2,900 people since mid-March but warned that the figure could rise to take into account those reported missing.
The clashes with the defectors took place after troops on Thursday stormed villages west of Jabal al-Zawiya, near the Turkish border, the Syrian Observatory for Human Rights said.
"Seven soldiers and five deserters or civilians were killed in the clashes," the Britain-based Observatory said, adding that 32 people had been wounded.
The Observatory and the Local Coordination Committees, an activist network opposed to Assad's regime, said soldiers and security forces backed by tanks had raided the villages in Idlib province.
Four people died in similar clashes earlier in the week between Syrian troops and deserters -- who had defected after refusing to fire on anti-regime protesters -- also near the Turkish border, rights activists said.
An army officer who has taken refuge in Turkey, Colonel Riad al-Asaad, claims to have established an opposition armed force called the "Syrian Free Army", but its strength and numbers are unknown.
Analysts warn that the unrest in Syria, which began in mid-March as peaceful protests against Assad's autocratic rule, is becoming increasingly armed in response to the relentless gunning down of protesters by security forces.
The UN raised the death toll by 200 from its last estimate of 2,700 people killed at the beginning of September.
"The total number of people killed since protests began in Syria now stands at more than 2,900," UN High Commissioner for Human Rights spokesman Rupert Colville said Thursday, a day before the rights body was due to discuss the crisis.
Pro-democracy activists called for fresh demonstrations on Friday under the banner: "The Syrian National Council is our representative, mine, yours and that of all Syrians."
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In a video posted on YouTube, historic opposition figure Riyad Turk, 71, also endorsed the Syrian National Council formed in Istanbul to represent the main currents standing against the Assad regime.
"We call on all the forces of the revolution to join together and call for the regime's end and the establishment of a democratic and civil state," said the former leader of the Syrian Communist Party who spent 20 years behind bars.
Fears that the violence linked to the anti-regime revolt is spilling across the borders were heightened on Thursday. According to a security official, Syrian troops entered Lebanese territory and shot dead a Syrian farmer living in a remote border area of the eastern Bekaa region.
"At around 2:30 pm (1130 GMT), in an area called Saaba, next to Aarsal, Syrian troops entered Lebanese territory and opened fire on farmer Ali al-Khatib, a Syrian national," the Lebanese official told AFP, speaking on condition of anonymity.
He said Khatib was married to a Lebanese and lived in the area, where the border is not clearly delineated.
A government official, also speaking on condition of anonymity, confirmed the incident and said it was unclear why Khatib was targeted or how he was killed.
Earlier this week, Syrian tanks entered the same region in a brief incursion.
Diplomats in Brussels said the EU was preparing to target a Syrian commercial bank and 29 Iranians accused of human rights violations in fresh sanctions to be announced in the coming days.
One EU diplomat named the bank as the Commercial Bank of Syria, targeted by a US assets freeze in August.
The 27-nation bloc last month banned the delivery to Syria's central bank of bank-notes and coins produced in the EU in a seventh round of sanctions designed to step up economic pressure on Assad's regime.
The planned new measures come after European nations failed in their attempt to push a resolution through the UN Security Council on Tuesday. It had called for "targeted measures" against Damascus if it did not halt its crackdown.
The resolution was vetoed by Russia and China, provoking the fury of the United States and EU powers.
Syria, which maintains that the regime's opponents are armed gangs and terrorists trying to sow chaos, hailed the vetoes as "historic."
Following the lost vote at the Security Council, the British, French and German foreign ministers all said new pressure would be put on the Syrian government.