(Updates with comments, changes headline)
WASHINGTON, March 21 (Reuters) - It increasingly appears that a chemical weapon was not used in Syria this week, a U.S. official said Thursday, although officials cautioned that U.S. intelligence agencies have not yet reached a final conclusion.
"Our growing sense is that weaponized CW was not used," the official said, speaking on condition of anonymity. Still, the official left open the possibility that information could arise that changed the analysis.
Syrian President Bashar al-Assad's government and rebels accused each other of using chemical weapons in a rocket attack near Aleppo on Tuesday that killed 26 people.
A European security official said that if chemical weapons or other "weapons of mass destruction" had been fired off, the casualty toll would be much higher than 26.
The official said he did not believe that the evidence showed chemical weapons had been used.
After the attack on Tuesday, some of those hospitalized told a Reuters photographer they detected a strong smell of chlorine in the air and that many victims had fallen down dead after the blast.
President Barack Obama said in Israel on Wednesday that Assad would be held accountable if it were determined that chemical weapons had been used in Syria.
On Thursday a U.S. intelligence official said, "The intelligence community has not made an assessment as to whether or not chemical weapons were used or not."
U.S. Ambassador to the United Nations Susan Rice issued a statement on Thursday welcoming the announcement of a U.N. investigation.
"As the U.N. proceeds with these efforts, we will also continue to work closely with our partners to obtain further information regarding any and all credible allegations of the potential or actual use of chemical weapons in Syria," she said.
Two senators wrote a letter to Obama saying that more must be done to stop the killing in Syria and force Assad to relinquish power.
"The potential use of chemical weapons only makes the case for greater action more compelling and urgent," said the letter from Senate Armed Services Committee Chairman Carl Levin, a Democrat from Michigan, and Republican Senator John McCain of Arizona. (Reporting by Tabassum Zakaria and Phil Stewart; Editing by Stacey Joyce and Xavier Briand)