International peace envoy Lakhdar Brahimi was in Istanbul on Saturday for talks with Turkish officials as tensions soared between Damascus and Ankara over cargo seized from a Syrian passenger plane.
The United States insisted Turkey had been right to intercept the "serious military equipment" on the flight from Moscow to Damascus on Wednesday but Russia expressed outrage saying the cargo was entirely legitimate.
Brahimi, the veteran Algerian diplomat who is envoy of the United Nations and the Arab League, arrived in Istanbul from talks in Saudi Arabia.
The Damascus regime accuses both governments of arming the rebels who have been seeking to overthrow it in a conflict that has cost more than 32,000 lives in the past 19 months, according to human rights monitors.
A protest against President Bashar al-Assad in the Syrian city of Aleppo on Friday. International peace envoy Lakhdar Brahimi was in Istanbul on Saturday for talks with Turkish officials as tensions soared between Damascus and Ankara over cargo seized from a Syrian passenger plane.
Tensions between Damascus and its larger northern neighbour have soared since a shell fired from the Syrian side killed five Turkish civilians in a border village on October 3.
The Syrian army has been battling to restore its supply lines between the capital and second city Aleppo since rebels captured a key town on the main highway earlier this week.
Rebel fighters in the northern metropolis have been taking on government troops in the city's historic heart, doing battle even inside its landmark Umayyad Mosque.
Nationwide, the army has lost more than 130 soldiers in two days, according to figures from the Syrian Observatory for Human Rights. More than 250 more have been taken prisoner.
A total of 151 people were killed on Friday, the Britain-based watchdog said -- 61 soldiers, 50 civilians and 40 rebels.
Washington acknowledged that Moscow had broken no rules in allowing the cargo on board the flight to Damascus but said that it was morally reprehensible for maintaining its longstanding alliance with the government of President Bashar al-Assad.
"We have no doubt that this was serious military equipment," said State Department spokeswoman Victoria Nuland.
She acknowledged that Russia had not violated any embargo on Assad's regime but said "the policy's still morally bankrupt".
"Everybody else on the Security Council is doing what it can unilaterally to ensure that the Assad regime is not getting support from the outside," she said
The Syrian army has suffered major setbacks in the north of the country, its resources worn down by an increasing number of battle fronts and rebel attacks, analysts say. Duration: 00:22.
"We have been saying for almost a year now, that no responsible country ought to be aiding and abetting the war machine of the Assad regime."
Russian Foreign Minister Sergei Lavrov said the Syrian Air flight had been carrying a cargo of radar equipment, which could have either civilian or military uses but insisted Moscow had violated no laws.
"This cargo is electrical technical equipment for radar stations, this is dual-purpose equipment, but is not forbidden by any international conventions," Lavrov said.
Moscow and Beijing have repeatedly vetoed moves at the UN Security Council to take action against the Assad regime, angering Western governments.
As fighting raged on the ground, some of it close to the border, the tensions between Syria and Turkey grew.
Turkey scrambled a fighter jet on Friday after a Syrian helicopter attacked the rebel-held town of Azmarin near the border, an official in Ankara told AFP.
Top diplomats rushed to try to contain the fallout. German Foreign Minister Guido Westerwelle was also due in Istanbul on Saturday for talks with his Turkish counterpart Ahmet Davutoglu.
"It is important that no one pours oil on the fire. We are counting on moderation and de-escalation," Westerwelle said.
Syrian jets bombed rebel fighters attacking a key army base on the main road between Damascus and Aleppo early Saturday, the Syrian Observatory said. At least 20 rebels were wounded.
The Wadi Deif base is the army's most important garrison around Maaret al-Numan, a strategic town on the highway which the rebels seized on Tuesday.
South of the town, government jets launched a series of air strikes late on Friday killing 12 rebels, the Observatory said.
As the fighting raged, concern mounted over the humanitarian cost.
After his talks with Saudi King Abdullah on Friday, Brahimi's spokesman Ahmad Fawzi said the envoy and the king agreed "on the dire need to stop the bloodshed and provide humanitarian aid to the more than 2.5 million Syrians" affected.