WASHINGTON (Reuters) - The United States has indications that a toxic chemical, probably chlorine, was used in Syria this month and is examining whether the Syrian government was responsible, the U.S. State Department said on Monday.

"We have indications of the use of a toxic industrial chemical" in the town of Kfar Zeita, State Department spokeswoman Jen Psaki said, referring to a rebel-held area.

"We are examining allegations that the government was responsible," she told a regular news briefing. "Obviously there needs to be an investigation of what's happened here."

Syrian opposition activists reported that helicopters dropped chlorine gas on Kfar Zeita on April 11 and 12. The U.S. ambassador to the United Nations, Samantha Power, told ABC television's "This Week" on April 13 that the attack was "unsubstantiated."

Psaki said chlorine was not one of the priority one or two chemicals Syria declared to the Organization for the Prohibition of Chemical Weapons (OPCW) under a Russian-U.S. agreement for the destruction of Syria's chemical weapons stockpile.

Psaki said the United States was still trying to determine the facts. "We take all allegations of the use of chemicals in combat use very seriously," she said." We'll work with the OPCW, who is obviously overseeing the implementation, and determine if any violation occurred."

A U.N. inquiry found in December that sarin gas had likely been used in Jobar, on the outskirts of Damascus, in August and in several other locations, including in the rebel-held Damascus suburb of Ghouta, where hundreds of people were killed.

The Ghouta attack caused global outrage and a U.S. threat of military strikes that was dropped after Syrian President Bashar al-Assad pledged to destroy his chemical weapons arsenal.

The Syrian government failed to meet a February 5 deadline to move all of its declared chemical substances and precursors, some 1,300 metric tons, out of the country. It has since agreed to remove the weapons by late April.

'WAR CRIMES'

Some U.S. lawmakers who have expressed deep skepticism about the chemical weapons agreement said the report, if verified, backed their long-standing call for President Barack Obama's administration to provide more support for the Syrian rebels.

"The Assad regime continues to carry out war crimes in its slaughter of innocent men, women, and children. Its breach of the chemical weapons agreement should surprise no one, and unless the Obama administration is willing to force a price for such behavior, we should only expect more atrocities to come," Republican U.S. Senator John McCain of Arizona, a frequent critic of Obama's foreign policy, said in a statement.

Rebel activists posted photographs and video they said showed an improvised chlorine bomb to back up their claims about Kfar Zeita. The government accused rebels of using the chemical.

Asked about the government charge, Psaki said:

"We're examining allegations. We're obviously looking at the facts on the ground. We shouldn't forget the context of what the regime has been capable of in the past."

Psaki rejected presidential elections announced by Syria on Monday as "a parody of democracy" with no credibility.

"Staging elections under current conditions, including the effective disenfranchisement of millions of Syrians, neither addresses the aspirations of the Syrian people, nor moves the country any closer to a negotiated political solution," she said.

Syria announced a presidential election for June 3, preparing the ground for Assad to defy widespread opposition and extend his grip on power, days after he said the civil war was turning in his favor.

Last week, opposition activists accused Assad's forces of a new poison gas attack in the Syrian capital and posted footage of four men being treated by medics.

They said this chemical attack, the fourth the opposition has reported this month, was in the Harasta neighborhood of Damascus.

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