The hope is to finalize the designation before the Friends of Syria meeting, which is slated to be held Wednesday in Morocco.
The goal of the designation is to isolate extremists groups in Syria while giving a boost to the new political opposition group unveiled last month in Doha, Qatar, they said.
Al-Nusra and several other groups announced their opposition to a new anti-government coalition last month. U.S. officials estimate al-Nusra members represent some 9% of rebel forces in Syria.
Officials said the group has not affiliated itself publicly with al Qaeda in an apparent effort to appear more mainstream.
In recent months, the al-Nusra Front has emerged as one of the most effective groups in the Syrian resistance, drawing on foreign fighters with combat experience in Iraq and elsewhere.
It has claimed responsibility for a number of complex attacks in Damascus and Aleppo, frequently involving suicide bombers.