(CNN) - As accusations fly over chemical weapons in Syria, some are skeptical about whether a chemical assault has already happened, and intelligence officials around the world are investigating, U.S. sources said Wednesday.
Investigators are talking to rebels and defectors, poring over medical intelligence regarding symptoms reported by doctors, and looking at satellite imagery used to track missiles launched and chemical weapons movements, the sources told CNN.
The question of whether chemical weapons have been used in Syria came as President Barack Obama arrived Wednesday in Jerusalem to meet with Israeli Prime Minister Benjamin Netanyahu.
"That will be a very high item on the agenda," Netanyahu spokesman Mark Regev told CNN. "We don't want to see chemical weapons fall into the hands of dangerous terrorists that are all over Syria."
But Regev said the Israeli government had no confirmation about the use of chemical weapons.
Officials disagree over whether chemical weapons used
Regev's comments did not square with those of Israeli Justice Minister Tzipi Livni.
"It is clear for us here in Israel" that chemical weapons have been used in Syria, and an international response should be on the table, Livni told CNN in an exclusive interview from her home in Tel Aviv.
Livni wouldn't say whether there is evidence the Syrian government has directed the use of any chemical weapons.
But she said the development poses a direct threat to Israel, which shares a border with Syria.
And Robert Ford, the American ambassador withdrawn from Damascus more than a year ago with the closing of the U.S. Embassy there, told a House hearing Wednesday there is "no evidence" to substantiate reports of chemical weapons but that U.S. officials were taking such accounts "very seriously and we are using all of our available means to determine what happened."
He added there would be "consequences" for any such use.
On Thursday, Ford returned to the region along with a U.S. delegation, touring a Syrian refugee camp in Turkey to bring more attention to the growing humanitarian crisis. As the civil war has intensified in Syria, hundreds of thousands of people have sought refuge in Turkey, Lebanon, Jordan and other neighboring countries.
Concerns centered on an attack Tuesday in Khan al-Asal in the northern province of Aleppo. State-run media blamed rebels for the attack, which it said killed 25 people and injured more than 110 others.
Rebels say they have no chemical weapons
But the opposition Free Syrian Army said rebels don't have access to chemical weapons and blamed the casualties on a government missile.
Separately, an opposition group said the government attacked the southern town of Ateibeh with "chemical rockets," causing an unspecified number of deaths along with cases of suffocation, nausea and hysteria. There was no immediate government response.
The reports ignited a firestorm of reactions around the world, with Russia slamming the rebels and some U.S. lawmakers saying that Washington might need to take action against the Syrian government.
"I have a high probability to believe that chemical weapons were used," House Intelligence Committee Chairman Mike Rogers, R-Michigan, told CNN on Tuesday. "We need that final verification, but given everything we know over the last year and a half, I ... would come to the conclusion that they are either positioned for use, and ready to do that, or in fact have been used."
Sen. Dianne Feinstein, D-California, chairwoman of the Senate Intelligence Committee, added, "I think it is a very serious situation. I think the president of Syria ought to know this, and I think that the White House needs to complete an assessment and make some statement."
Obama has identified use as a 'red line'
In August, Obama urged Syrian President Bashar al-Assad not to use chemical weapons, saying that would constitute crossing a "red line."
"We have been very clear to the Assad regime -- but also to other players on the ground -- that a red line for us is, we start seeing a whole bunch of chemical weapons moving around or being utilized," he told reporters. "That would change my calculus, that would change my equation."
Obama added that U.S. officials were monitoring the situation and had put together a number of contingency plans.
In December, he reiterated in a speech at the National Defense University in Washington that any use of chemical weapons by Syria would result in U.S. action.
"The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable," he said.
Then-Secretary of State Hillary Clinton also raised the matter. "I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people," she said. "But suffice it to say, we are certainly planning to take action if that eventuality were to occur."
White House Chief of Staff Denis McDonough on Tuesday told CNN that, if reports of chemical warfare are substantiated, "this is a game changer, and we'll act accordingly."
But images posted by Syrian state-run media of the aftermath of the Aleppo incident, which the government blamed on rebels, aren't consistent with a chemical weapon attack, some observers said.
"There are no images of the site of the attack; just of some affected people. These people do not show outward symptoms of a CW (chemical weapon) attack. Definitely not mustard; definitely not a nerve agent," wrote Jean Pascal Zanders, senior research fellow at the European Union Institute for Security Studies.
"There are far too many people, including non-medical staff, around the affected persons. Apart from a surgical mask, nobody wears any protective garment or gas masks. If there would have been a CW attack with one of the agents known (or believed) to be in Syria's arsenal, then most of the people present would have been fatally or seriously contaminated."
U.S. and Russia at odds
Two senior U.S. officials said they don't believe the rebels used chemical weapons and suggested the government may have manufactured the incident to preserve its ability to use them in the future.
"The regime is using (the claims) as a pretext for their own possible use," one of the officials said. "The opposition has no such weapons."
But the Russian Foreign Ministry, citing information from Damascus, said rebels did use chemical weapons, causing deaths and injuries.
"We believe the new incident is an extremely alarming and dangerous development in the Syrian crisis," the ministry said. "Russia is seriously concerned about the fact of (weapons of mass destruction) coming into the hands of militants, which makes the situation in Syria even worse and brings the confrontation in the country to a new level."
Not the first round of claims
U.S. officials pointed to previous claims that the Syrian government used chemical weapons, which were found to be false after extensive investigation.
The Syrian government did not use chemical weapons against residents of Homs in a December attack, a U.S. State Department investigation showed, but did apparently misuse a riot-control gas in the incident, according to senior U.S. officials.
The officials said the State Department launched a probe from its consulate in Istanbul, Turkey, following reports from doctors and activists that dozens of victims suffered from nervous system, respiratory and gastrointestinal ailments after inhaling the gas in Homs on December 23.
Military analysts believe the Syrian government may have one of the largest stockpiles of chemical weapons in the world.
The civil war -- which began two years ago after a government crackdown on Syrian protesters -- has left around 70,000 people dead and uprooted more than 1 million others, the United Nations has said.