Deadly fighting flared on Syria's border with Turkey on Monday, while Israel fired across the ceasefire line on the Golan for a second day, stoking fears of a spillover of the 20-month conflict.
Unlike on Sunday, when Israeli troops fired only a warning shot, the Israeli army said it scored "direct hits" on the source of the new mortar fire which struck the part of the Golan it occupies.
But even the earlier fire -- Israel's first across the UN-monitored ceasefire line since the 1973 Middle East war -- left UN chief Ban Ki-moon "deeply concerned by the potential for escalation," his spokesman Martin Nesirky said.
Air strikes and shelling of rebel positions in the town of Ras al-Ain on the Turkish border killed at least 12 people, the Syrian Observatory for Human Rights said.
One bomb exploded less than 150 metres (yards) from the border, with the blast blowing out windows in houses in Ceylanpinar, where there were also injuries, the Turkish town's mayor told Anatolia news agency.
That blast killed four Syrians and wounded scores of others, around 20 of them seriously, the agency said, later adding that six of the wounded died of their wounds in hospital.
The Observatory said seven rebels and five civilians died in the raids.
The air strikes sent a new wave of civilians pouring across the border to the Turkish side, adding to the 9,000 refugees who already fled late last week, an AFP photographer said.
A food factory seized by rebels was targeted, Anatolia said.
Near the town, helicopters strafed rebel forces who for the past three days have besieged an army post manned by 75 soldiers, said the Observatory which relies for its information on a network of activists and medics on the ground.
At the far eastern end of Syria's border with Turkey, Kurdish militia took control of the village of Malikiya after troops loyal to President Bashar al-Assad withdrew, the Observatory said.
It was the latest in large swathes of territory in Kurdish-majority areas of the northeast and the northwest to escape central government control without going over to the Arab-led rebels.
The exchange of fire on the Golan came after renewed clashes between loyalist troops and rebels on the Syrian-controlled side of the strategic plateau resulted in the mortar round being fired across the UN-monitored ceasefire line.
Three rebel fighters were killed in the clashes, the Observatory said.
The Israeli military said: "A mortar shell hit an open area in the vicinity of an IDF (army) post in the central Golan Heights, as part of the internal conflict inside Syria, causing no damage or injuries.
"In response, IDF soldiers fired tank shells towards the source of the fire, confirming direct hits."
Israeli military sources said: "Syrian mobile artillery was directly hit."
On the Jordanian frontier, shelling and clashes with rebels left two Syrian border guards dead, the Observatory said.
Warplanes also bombed rebel positions on the strategic highway between Damascus and second city Aleppo, while clashes on the southern outskirts of the capital near a Palestinian refugee camp killed seven civilians, it added.
The Observatory said 13 soldiers were killed in a rebel ambush in the Homs region of central Syria, while warplanes also launched raids in the provinces of Damascus, Raqa, Hasakeh, Idlib and Deir Ezzor.
At least 52 people were killed across Syria on Monday, including 24 civilians, said the Observatory, which has given an overall death toll of more than 37,000 since the revolt broke out in March 2011.