Mitri said he welcomed a decision by Libya's General National Congress to form a committee tasked with drafting a new constitution.
The decision, he said, is an important milestone that helps Libya toward political reform.
"We hope that the General National Congress moves swiftly to adopt the legal framework necessary for free, fair, and inclusive elections of the Constituent Assembly," he said in a statement.
Mitri told members of the U.N. Security Council last month there were many difficult decisions left to make for the post-war Libyan government. Most notably, he said, was security sector reform.
Libya has ramped up security in recent days in response to a terrorist attack on a natural gas installation in neighboring Algeria. Attacks on Western diplomatic outposts in Benghazi, Libya, raised concerns about the country's stability two years after civil war.
Libyan leader Moammar Gadhafi's death in October 2011 ended a generation-long dictatorship.