U.N. Secretary-General Ban Ki-moon received a formal request in March from Syrian Ambassador to the United Nations Bashar Ja'afari to look into claims anti-government forces used chemical weapons in Aleppo in mid-March.
Ban said then he was aware of "other allegations of similar cases" made during the Syrian civil war.
He said, however, that he's received word the Syrian government was objecting to a U.N. probe on the alleged use of chemical weapons.
"I appeal to the government of Syria to extend its fullest cooperation, and to allow the investigation to proceed," he said in a statement.
Syria's state-run news agency SANA reported the government suspected the United Nations was trying to broaden the scope of its investigation.
Ban appointed Ake Sellstrom, a former U.N. weapons inspector in Iraq, to lead a fact-finding mission to investigate the use of chemical weapons in Syria.
The Syrian civil war is in its third year. The United Nations estimates that at least 70,000 people died as a result of the war and another 1 million people are displaced.
U.S. Secretary of State John Kerry is expected to meet with members of the Syrian opposition during meetings this week in London. State Department spokesman Patrick Ventrell said the U.S. government was "analyzing all options" for supporting opponents of the Syrian government.