Syrian rebels have held out for another night in the city of Aleppo as a government offensive enters its fourth day.
With battles raging in several areas, hospitals and clinics in the city are said to be filling up with casualties.
Thousands of people are continuing to flee the city, adding to a growing humanitarian crisis in the region.
Meanwhile, the US and Turkey have agreed to step up efforts to achieve "political transition" in Syria.
This would include the departure of Syrian President Bashar al-Assad, the White House said.
In a phone call on Monday, US President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan also agreed to help the growing numbers of refugees - both inside Syria and in neighbouring countries.
Turkey has been a staunch critic of President Assad and has given refuge to large numbers of army defectors.
Thousands of Syrian refugees are also living in refugee camps on the Turkish side of the countries' long border.
As the crisis deepened, Iran - a close ally of Damascus - warned Turkey not to intervene militarily, the Syrian state-controlled al-Watan newspaper said.
"Any attack on Syrian territory will meet with a harsh response, and the Iranian-Syrian mutual defence agreement will be activated," it reported on Monday.
The newspaper said Turkey and the US were planning to create "a safe haven guarded by the armed gangs" in the north of Syria.
"Turkey has received very strong warnings in the past few hours and the following message - beware changing the rules of the game," al-Watan said.
Turkey has said it will use troops if necessary to prevent another Halabja - a reference to a notorious massacre in Saddam Hussein's Iraq.
In Aleppo, rebels said they were holding out in the strategic south-west area of Salah al-Din, rejecting government claims to have retaken it.
Pro-government forces have been attacking with artillery, ground forces and helicopter gunships.
Analysts say Salah al-Din provides an important route for Syrian troop reinforcements coming from the south.
"We always knew the regime's grave would be Aleppo," said one young fighter named only as Mohammed, quoted by Reuters news agency.
"Damascus is the capital, but here we have a fourth of the country's population and the entire force of its economy. Bashar's forces will be buried here."
A medic in one clinic in Aleppo told Reuters: "Some days we get around 30, 40 people, not including the bodies. A few days ago we got 30 injured and maybe 20 corpses, but half of those bodies were ripped to pieces. We can't figure out who they are."
The BBC's Ian Pannell, in the Aleppo area, says it is a vital city that neither side can afford to lose.
Vital supplies are running out in certain areas, he says, and those staying behind face long queues for bread.
Cars and lorries can be seen carrying people out the city, but most of those fleeing are women and children, with the men opting to stay and fight, our correspondent adds.
Greek officials were due to reveal further details of new border security plans on Tuesday, aimed at stopping Syrian refugees crossing from Turkey.
On Monday, Greece - one of the busiest entry points for illegal immigrants entering the EU - said it was quadrupling the number of guards along its border with Turkey.
In New York, UN Secretary General Ban Ki-moon repeated that he was particularly concerned by the continued use of heavy weapons by Syrian government forces.
He also called on the Syrian government to renounce the possibility of using chemical weapons under any circumstances, and to secure its stockpiles.
Damascus has implicitly acknowledged its possession of chemical weapons but said it would not use them against its own people, only against foreign invaders.
