Some 230,000 Syrians have fled their homes and taken refuge in other parts of the country since the March 2011 start of the uprising against President Bashar Assad, according to a U.N.-backed report.

But unlike the tens of thousands of other Syrians who have crossed into neighboring countries, aid groups cannot reach many of those displaced inside Syria, the head of the U.N. refugee agency said Thursday.

"The U.N. country team has been asking repeatedly for access," the U.N.'s High Commissioner for Refugees, Antonio Guterres, told reporters in Geneva.

A report by the Norwegian Refugees Council and the Geneva-based Internal Displacement Monitoring Center estimates Syria now has more than 600,000 internally displaced people, one of the highest numbers in the world.

Before the uprising began, Syria had about 400,000 internally displaced people, mostly from its 1967 war with Israel.

Last year across the world, 3.5 million people were forced from their homes by conflicts.

Overall, Colombia has the highest number of internally uprooted people, up to 5.2 million, followed by Iraq and Sudan, each with more than two million displaced. Congo and Somalia also had over a million uprooted civilians last year.

Under international law, internally displaced people are different from refugees, who enjoy greater protection once they flee their home country.