Lebanon's prime minister, Fouad Siniora made the request shortly after a car bomb Monday killed an anti-Syrian lawmaker and newspaper publisher, Gebran Tueni in Beirut.
The U.N. Security Council is expected to decide Thursday on a proposal to expand an existing U.N. probe into the February death of former Prime Minister Rafik Hariri to include other political attacks.
The proposal would also give a six-month extension to the Hariri probe, which is set to expire this week.
In Washington, a State Department spokesman, Sean McCormack said the extension is crucial to finish the work of the Hariri probe, which he said has been slowed by lack of Syrian cooperation.
Syria has rejected the claims and repeatedly denied any role in the Lebanon killings.