The UNHCR says it has accounted for 509,559 refugees so far, primarily in Lebanon, Jordan and Turkey, and that many more are yet to come forward.
More than 2m people are also thought to be internally displaced within Syria.
Meanwhile, a UK-based activist group says rebels led by an Islamist faction have seized an army base near Aleppo.
A video posted online on Monday showed rebels overrunning the Sheikh Suleiman base near the key northern city of Aleppo and seizing military vehicles.
The base is now fully under the control of rebels led by the Islamist al-Nusra Front, the Syrian Observatory for Human Rights said.
Correspondents say the sprawling base was the only significant government facility remaining in a broad stretch of countryside west of Aleppo.
Western nations have expressed concern at the role of Islamist groups like the al-Nusra Front in the insurgency in Syria.
On Monday, US Secretary of State Hillary Clinton designated the al-Nusra Front a foreign terrorist organisation, freezing any assets its members may have in US jurisdictions.
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The designation is largely symbolic because the group is not thought to have holdings or support in the United States, but officials hope the penalties will encourage others to take similar action and discourage Syrians from joining.
Activists also reported continued government bombardment and fighting in outlying areas of the capital Damascus, which has seen fierce clashes in recent weeks.
The UNHCR says Lebanon is playing host to most refugees, with 154,387 Syrian refugees either registered or in the process of being registered there.
Jordan has received 142,664, while there are in 136,319 in Turkey, 64,449 in Iraq and 11,740 in North Africa, the agency says.
"Syrian refugees arriving during recent bad weather reached Jordan with soaked clothing and mud-covered shoes due to heavy rainfall," UNHCR chief spokeswoman Melissa Fleming said.
"Protection teams described the night-time arrivals as fearful, freezing and without proper winter clothing," she added.
Since the beginning of November, 3,200 new Syrian refugees have been registered every day in the region, although some of these are thought to be people who had been in the host countries for some time but had not sought help.
Most Syrian refugees were currently living outside camps "often in rental housing, with host families, or in various types of collective centres and renovated accommodation," the agency said.
"The numbers of those struggling to live on the local economy and who eventually come forward to register are expected to increase," it added.

The 44-10 win sees Ireland go top of Pool D and they can seal a quarter-final spot if they beat Italy at the Olympic Stadium next Sunday.
"Next week is the pivotal week. It's pressure for us and pressure for Italy," said Schmidt.
"That is why people come to a World Cup - they want to see how teams respond."
After facing Italy, Ireland complete their pool fixtures with a match against France on 11 October.
"For us it starts in earnest now, because we know we can qualify if we win next weekend," added Schmidt.
"The game against France just decides who we play in the quarter-final if we can win next week.
"Italy, after a win and a loss, need to win to qualify. It becomes a last-16 game for us."
Ireland must wait to find out if Rob Kearney will be fit after he was injured in scoring the fifth try shortly after coming on as a substitute.
"Rob was involved in a high-speed play after coming on the pitch and just after scoring the try he's jarred himself," Schmidt added.
"We hope it is not too much but the next 24 to 48 hours will tell if he has done anything telling for next week."
Losing Kearney would be a setback for Ireland, but several players impressed in a dominant performance against Romania.
Schmidt made 12 changes with Italy and France in mind, but his altered line-up showed few signs of weakness, scoring six tries, including four in the second half.
Tommy Bowe and Keith Earls crossed the line twice each, before Kearney and Chris Henry added two more late on, and Simon Zebo also staked a claim with an impressive display at full-back.
"There were a number of players who performed really well and will create a big of selection headache for Italy," Schmidt said.
More than half a million people have now fled the conflict in Syria to neighbouring countries, according to the UN's refugee agency.
[NEXT_CONCEPT]
Ireland's World Cup campaign "starts in earnest now" after Sunday's convincing bonus-point victory over Romania, according to coach Joe Schmidt.