The UNHCR also forecast that the number of Syrian refugees could shoot up to 4.27 million by December from the current figure of more than three million.
The mid-year review came as the head of the UN refugee agency warned of the worst displacement problem in seven decades.
"The Syria and Iraq mega-crises, the multiplication of new crises and the old crises that seem never to die have created the worst displacement situation in the world since World War II," UNHCR chief Antonio Guterres said on Tuesday.
He said the number had topped 50 million for the first time since 1945.
The report said Syrians had for the first time become the largest refugee population under its mandate, overtaking Afghans who had held that position for more than 30 years.
The UNHCR ranking however does not include Palestinians, who are covered by another UN agency. There are an estimated five million Palestinian refugees around the world.
After Syria and Afghanistan (2.7 million), the leading countries from where refugees come include Somalia (1.1 million), Sudan (670,000), South Sudan (509,000), the Democratic Republic of Congo, Myanmar and Iraq.
"At more than three million as of June 2014, Syrian refugees now account for 23 percent of all refugees being helped by UNHCR worldwide," it said.
According to UNHCR, more than 1.4 million people were newly dispaced across borders in the first half of last year, with Syrians accounting for half this number at 704,400.
The Syrian conflict has killed more than 200,000 people since March 2011 and displaced around half the country's population.
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SYRIANS TAKE HUGE RISKS
The protracted conflict has seen Syrians flocking to neighbouring countries such as Lebanon, Turkey and Jordan but also increasingly taking hazardous sea journeys across the Mediterranean in a desperate bid to get to Europe.
The latest example of this was the case of 768 Syrian migrants who were found drifting towards the Italian coast last week in an old cargo ship after being abandoned by people traffickers.
Although Pakistan continued to host the largest number of refugees worldwide at 1.6 million -- virtually all Afghan -- the largest number of new refugee arrivals between January and June 2014 was reported by Lebanon.
"With an increase of about 325,000 Syrians during the first half of the year, by mid-2014 (Lebanon's) registered refugee population passed the 1.1 million mark," it said.
UNHCR said Lebanon shot up from being the 69th largest refugee-hosting country to second largest within just three and a half years.
It appealed for more international aid and support to help countries and local communities overwhelmed by the new arrivals.
"The economic, social and human cost of caring for refugees and the internally displaced is being borne mostly by poor communities, those who are least able to afford it," Guterres said in the report.
"Enhanced international solidarity is a must if we want to avoid the risk of more and more vulnerable people being left without proper support," he said.
UNHCR said that by mid-2014, it was helping around 46 million displaced people, including 13 million refugees -- the highest number since 1996.