Turkey bombarded Syrian army positions afresh on Monday in response to what Turkish officials said was a new shell strike on a border town, just hours after UN chief Ban Ki-moon warned of the dangers.
Ban said there was a growing risk that the conflict in Syria, now in its 19th month, could spill over into neighbouring Turkey and Lebanon, and called for an end to foreign arming of either side.
But in the United States, barely four weeks before the presidential election, Republican challenger Mitt Romney accused incumbent Barack Obama of "sitting on the sidelines" of the Syrian conflict and vowed that, if elected, he would ensure the rebels obtained the weapons they needed.
On the ground, the Syrian army pressed an offensive against rebel-held areas of the central province of Homs, seeking to eliminate the last pockets of resistance to free up troops for the north and the key battleground city of Aleppo.
The Syrian shell struck in the Altinozu district of Turkey's Hatay province, at the western end of the two countries' border, a Turkish official said, speaking on condition of anonymity.
He said troops had been under orders to respond to all cross-border fire since shelling from the Syrian side killed five Turkish civilians, including a mother and her three children, on Wednesday last week, sending tensions soaring.
"The Turkish military retaliates immediately after every single Syrian shell," the official said.
The Turkish parliament on Thursday gave the government the green light to use military force against Syria if necessary.
Turkish President Abdullah Gul said that was not a mandate for war but on Monday said he remained in constant contact with military commanders to ensure an appropriate response.
"Whatever is necessary is being done as you already see, and will continue to be done," Mr. Gul said.
The UN Security Council on Thursday strongly condemned cross-border fire by Syria and called for restraint between the two neighbours.
On Monday, Mr. Ban warned: "The escalation of the conflict along the Syrian-Turkish border and the impact of the crisis on Lebanon are extremely dangerous.
"I am deeply concerned by the continued flow of arms to both the Syrian government and opposition forces. I urge again those countries providing arms to stop doing so," he added.
The Syrian government accuses Turkey and Gulf states Qatar and Saudi Arabia of backing the rebels. The Syrian opposition charges that President Bashar al-Assad's regime is receiving support from his close ally Iran.
In a key foreign policy address on Monday, Mr. Romney accused Obama of weak leadership in the Syrian crisis which he said was emblematic of an administration that opts to "lead from behind." "In Syria, I will work with our partners to identify and organise those members of the opposition who share our values and ensure they obtain the arms they need to defeat Assad's tanks, helicopters, and fighter jets," the Republican candidate pledged.
On the ground, the army attacked rebel neighbourhoods of the city of Homs – Syria's third largest – and the nearby town of Qusayr, where rebel forces have been under siege since late last year, sources on both sides said.
"The army is in the midst of trying to cleanse the last rebel districts of the city of Homs," a Syrian army commander told AFP.
"The army has already cleansed the villages surrounding Qusayr, and is now trying to take back the town itself," the commander said on condition of anonymity.
A security official told AFP the army hopes to retake the besieged areas by the end of the week to free up troops for battle zones in the north, such as Aleppo.
"It is a huge operation, and we hope to finish it off by the end of this week," the source said, speaking on condition of anonymity.
"After that, we will concentrate on the north of Syria." Homs province has suffered some of the worst bloodshed and destruction of the uprising which erupted against Assad's regime in March last year, but since July the main focus of the conflict has shifted to Aleppo, the northern metropolis of some 1.7 million people.
The army pounded a string of rebel-held neighbourhoods in the north and east of Aleppo on Monday.
Hospital staff in just one of those districts – the Shaar neighbourhood – told AFP they had received seven dead – four civilians and three rebel fighters – and 55 wounded.
Nationwide, a total of 141 people were killed on Monday – 56 civilians, 53 soldiers and 32 rebels, the Syrian Observatory for Human Rights said.
The death toll since the start of the uprising now tops 32,000, the Britain-based watchdog said.