The UN is to send an inspection team to Lebanon to check whether Damascus has withdrawn all its military and intelligence forces from the country.
Many believe Syria was behind the bomb blast that killed Hariri
The decision follows US complaints over Syrian "interference and intimidation" inside Lebanon after a formal pullout at the end of April.
Syria rejected the charge, insisting that all Syrian troops and intelligence officers were withdrawn on time.
Syria operated in Lebanon for 29 years, but left amid international pressure.
Earlier, President Bush voiced US suspicions over Syria, saying that Lebanon could not be free until all intelligence personnel left the country.
White House spokesman Scott McClellan also backed UN plans to send a monitoring team back to Lebanon.
Since April "Syria has not fully complied with the Security Council resolution and the demands of the international community" to withdraw all its forces from Lebanon, Mr McClellan said.
Syrian intelligence operatives "are interfering in Lebanon's internal affairs", he added, but said he would not comment further on "intelligence matters".
Earlier this week, UN Secretary General Kofi Annan said he had received reports that elements of Syrian intelligence may still be in Lebanon.
The UN passed Resolution 1559 last year, which called for the withdrawal of non-Lebanese forces.
The UN special envoy charged with monitoring the implementation of 1559 will go to Syria to meet President Bashar al-Assad in the next few days.
Damascus decided to pull out after the February assassination of former Prime Minister Rafik Hariri, which prompted a political crisis in Lebanon.
Opposition politicians and many Lebanese believe Syria was behind the assassination, but Damascus rejects the charge.
It has also denied involvement in the killing of a prominent anti-Syrian journalist, Samir Qasir, in Beirut last week.