A Syrian government official said in a phone call on Wednesday that al-Assad "will not lead the Syrian delegation to the UN summit" taking place 14 to 16 September.
The official spoke on condition of anonymity and gave no reason for the move.
The announcement contradicted last month's statement by Syria's UN Ambassador Fayssal Mekdad, who said that al-Assad would attend. But on Tuesday, Mekdad said al-Assad's participation had not been confirmed, suggesting that the president was having second thoughts.
In recent weeks, pressure has increased on Syria over the UN investigation into the February assassination of former Lebanese prime minister Rafik al-Hariri.
UN officials, including chief UN investigator Detlev Mehlis, have accused Syria of slowing the inquiry through a lack of co-operation. The UN Security Council, which mandated the investigation, was told that Syria had ignored requests to interview its officials and to hand over documents.
Syrian generals suspected
Last week, Lebanese authorities accused four pro-Syrian generals, who ran Lebanon's security services when al-Hariri was killed, of responsibility for the assassination.
In addition, the United States is putting pressure on Syria to make greater efforts to counter would-be insurgents infiltrating across its border into Iraq and to crack down on Palestinian resistance groups based in Syria.
Two senior US officials in Washington have said that US Secretary of State Condoleezza Rice will try to rally anti-Syrian support from European and Middle Eastern leaders during a meeting on the sidelines of the UN summit.
Lebanon's President Emile Lahoud will lead the Lebanese delegation to the UN, his office said after the announcement came from Damascus that Assad will not attend.
However, officials did not say whether Lahoud, a staunch ally of Syria, was invited to the meeting the Americans plan to hold on the sidelines of the UN summit to rally support against Damascus.
Lebanese President Emile
Lahoud is also under pressure
Syria's army and military intelligence were in control of Lebanon at the time of al-Hariri's assassination. The killing sparked massive anti-Syrian protests in Lebanon and greatly magnified the international pressure on Syria to withdraw its troops from Lebanon. The last troops left in late April.
Mehlis has been invited to go to Damascus and is scheduled to travel to the Syrian capital on Saturday.
Leaders of more than 170 countries are expected to attend the summit, which is intended to approve new policies on security, development, human rights and UN management.
Syria has never sent a head of state to such conferences in the past.