U.S. intelligence agencies are investigating whether chemical weapons were used this week in Syria but have not made a determination yet, a U.S. intelligence official said on Thursday.

“The intelligence community has not made an assessment at this point,” the intelligence official told Reuters on condition of anonymity. “The information has not pushed us far enough in one direction for us to make an assessment yet.”

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.

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.

U.S. Ambassador to the United Nations Susan Rice issued a statement on Thursday welcoming the announcement of a U.N. investigation.

“As the UN 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.

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.

Two senators wrote a letter to Obama saying “more active steps” were needed 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, and Republican Senator John McCain of Arizona.

(Reuters)