The United Nations is seeking $5 billion to fund relief operations in Syria through to the end of the year.
The UN launched the appeal -- its largest ever -- at an international conference in Geneva on June 7.
The UN had previously estimated that it would need $3 billion in humanitarian aid for Syria this year. So far, only $1.4 billion has been pledged.
UN refugee agencies have said that the updated figure is based on the worsening security situation in Syria, a sharp increase in the number of refugees, and the assumption that the conflict won't end soon.
The UN said it expects that a total 10.25 million Syrians inside and outside the country, or half of Syria's population, will need humanitarian aid by the end of 2013.
The number of Syrians who have fled to neighboring countries is expected to reach 3.45 million by the end of 2013, from 1.6 million now.
UN Emergency Relief Coordinator Valerie Amos spoke about the increased needs at a news conference in Geneva on Friday.
"We estimate that 6.8 million people [inside Syria] now need urgent help," said UN Emergency Relief Coordinator Valerie Amos at a news conference in Geneva on May 7. "That is one in three Syrians in need of urgent humanitarian assistance. And between January and April the number of people displaced within Syria more than doubled."
UN High Commissioner for Refugees Antonio Guterres maintained that failing to provide adequate humanitarian assistance to Syrians would only further increase the impact of the conflict on neighboring countries.
"This is not only a humanitarian need -- to support the people suffering enormously inside Syria and to support refugees outside -- it is also a key instrument to stabilize the region, to avoid this conflict [becoming] a much more dramatic, regional one."
The Syrian conflict began with peaceful protests against President Bashar al-Assad's regime in March 2011 but turned into a civil war.
The UN estimates more than 80,000 people have been killed since the conflict began.
With reporting by Reuters and AP