Stepped up fighting in Syria could create a "gigantic outflow" of people on a previously unseen scale, the UN's refugee commissioner said on Friday.
"We all have seen crises of this nature in different parts of the world, let's hope that this doesn't evolve into the kind of gigantic outflows we have witnessed in Afghanistan or Iraq," High Commissioner for Refugees Antonio Guterres told reporters in Geneva.
Reiterating his hope that a solution could be found to the civil war despite the resignation on Thursday of peace envoy Kofi Annan, Guterres said that those left inside Syria needed "absolutely life-saving work".
"Obviously, if the conflict intensifies the capacity to deliver will be more and more limited," he warned, adding that "contingency planning" was in place "for more dramatic situations".
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As many as 1.5 million people are internally displaced in Syria but getting access to them in conflict zones such as Aleppo remains the most serious problem, according to the UN refugee agency.
"It was impossible yesterday (Thursday) to send additional relief teams as the city was sealed off by military forces," said UNHCR spokeswoman Melissa Fleming.
UNHCR staff also reported a complete mobile telephone and Internet blackout in the city, Fleming said, adding that she was unable to provide any update on the situation on Friday.
"Terror is gripping the population and humanitarian aid is desperately needed", the UNHCR said in a statement on the crisis in Syria's second city, with thousands sheltering in schools, dormitories and mosques.
The agency also reported continuing threats to the refugee population in Damascus where a Sudanese had been shot in his legs by armed groups. On a single day some 700 refugees approached the UNHCR team in the country's capital, UNHCR said.
Tens of thousands of Syrians have already fled to neighbouring states, including Turkey, Lebanon and Jordan, while Guterres said some 10,000 Iraqis have decided to return home, despite the dangers in their own country.