Deadly fighting flared on Syria's borders with Turkey and Jordan on Monday while Israel fired across the ceasefire line on the Golan Heights 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 army said it landed "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 the UN chief "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 force of the blast blowing out windows in houses in the adjacent Turkish town of Ceylanpinar, where there were also casualties, the town's mayor told the Anatolia news agency.
That blast killed four Syrians and wounded scores of others, around 20 of them seriously, the agency said.
The air strikes sent a new exodus of civilians pouring across the border to the Turkish side adding to the 9,000 refugees who already fled late last week, an AFP
"There are wounded on the Syrian side but also in Ceylanpinar because of the windows blown out by the explosion," Mayor Ismail Arslan said.
"The ambulances are transporting the wounded without stopping," he said. "There is bomb damage everywhere."
The strike targeted a food factory which rebels seized in the past several days, Anatolia said.
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.
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."
The army warned any further fire from Syria towards the Israeli-occupied sector of the strategic plateau would be answered with "severity."
Israel had fired across the ceasefire line on Sunday but that was only a warning shot and the army said troops deliberately aimed to miss.
Even that exchange, however, prompted urgent appeals for calm from UN chief Ban Ki-moon.
"He calls for the utmost restraint" and urges both sides to uphold the 1974 accord which established the ceasefire line, Ban's spokesman said.
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 killed seven civilians, it added.
Fighter jets carried out five bombing runs on the rebel-held highway town of Maaret al-Numan, dropping barrels packed with explosives, Observatory director Rami Abdel Rahman told AFP.
The rebels' capture of Maaret al-Numan last month delivered a severe blow to the government's ability to reinforce its troops in Aleppo which have been under rebel attack since mid-July.