“We have reports we are trying to verify that thousands of Syrians overnight and yesterday crossed into Lebanon. So far reports vary between 8,500 and 30,000 people may have crossed in the past 48 hours,” said Melissa Fleming, chief UNHCR spokeswoman.
“Borders remain open...people continue to flood into Lebanon,” she said.
An estimated that one million people may have been forced to flee inside the country since the rebellion against authoritarian president Bashar al-Assad began in March 2011, according to the Syrian Arab Red Crescent.
A total of 120,000 Syrian refugees have sought protection in Jordan, Lebanon, Iraq and Turkey according to UNHCR registration statistics on 18 July, but host governments say the numbers are much higher.
Many newly arriving Syrian refugees are entirely dependent on humanitarian aid, with some coming with only the clothes on their backs, and following many months of unemployment.
"The needs of those who arrived earlier in the year are also increasing as their savings have become depleted," said UNHCR.
Local infrastructure in countries hosting the refugees has come under severe pressure especially water, housing, school and health facilities, the agency said.
UNHCR is also assisting 35,000 cash-strapped families in Syria and said the number of people seeking help with cash to rent apartments and buy basic household items is growing.
The UNHCR also said that Syrian banks are reported to be running out of cash and a rush to find safe housing has caused rents in some places to spike to 100 dollars per night.
"I fear for the civilians caught up in the violence in Damascus, including the large Iraqi refugee population residing there,” said UNHCR chief Antonio Guterres.
There are over 88,000 registered Iraqi refugees in Syria, most living in Damascus and around 8,000 refugees from other countries, UNHCR said.
On Tuesday the Iraqi government urges its citizen living in Syria to return home immediately amid an escalation of sectarian fighting in the conflict-wracked country.
Over 13,000 Iraqis left Syria in the first half of this year, most going back to Iraq.