Allegations are mounting that the Assad regime has returned to using chemical weapons against Syrian rebels and civilians, nearly two years after the government agreed to dismantle their stockpile. 

The suspicions have the Obama administration calling for an immediate U.N. investigation into the "abhorrent acts" -- though it remains unclear what, if any, punishment Bashar Assad might face if formally blamed for the string of alleged chlorine gas attacks. 

One western U.N. diplomat told Fox News the situation has become "unacceptable" in Syria. 

"There is mounting evidence of repeated chlorine attacks," the diplomat said. 

Civilians, including children, allegedly have been injured and killed in the latest attacks. In a letter sent this week to the U.N. Security Council from the National Coalition of Syrian Revolution and Opposition Forces, the group cited reports of chlorine gas attacks in the Idlib and Hama areas and urged the creation of a no-fly zone to protect the Syrian people. 

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"In the past two weeks alone, witnesses and medics on the ground in Idlib and Hama governorates reported at least nine separate instances of toxic chemical attacks -- several of them deadly," the group wrote. "... in each instance, barrel bombs loaded with poisonous chemical substances were deployed from Syrian regime helicopters." 

The allegations have U.S. and other western diplomats scrambling for answers, while making clear they expect the United Nations to probe the charges. The U.S. has submitted a preliminary draft Security Council resolution that aims to set up a mechanism for determining who is to blame and to hold them accountable. 

A U.S. official told Fox News the Security Council is overdue in addressing "the need to determine who is responsible" for the attacks. "Doing so is critical to getting justice for the Syrian people," he said. 

The U.S. proposal envisions the creation of a team of experts appointed by U.N. Secretary-General Ban Ki-moon that would have the know-how and support to get the access to investigate and then report its findings. 

Yet it's unclear what consequences the Assad regime might face if found responsible. Assad's veto-wielding ally on the Security Council, Russia, has historically blocked efforts to hold the Syrian government accountable for war crimes. 

Last week, State Department spokesman Jeff Rathke said officials are seeking more information about chlorine gas allegations but could not give specifics when asked repeatedly about potential consequences. "I don't have specific steps to announce now," he said. 

A State Department official told on Thursday they are still seeking more information about the allegations. 

"We continue to look very closely into this matter and are considering next steps.  ... if true, these attacks would be only the latest tragic examples of the Asad regime's atrocities against the Syrian people," the official said, adding that the international community "cannot turn a blind eye" to these actions. 

The official said the U.S. is "actively engaged" with U.N. colleagues and urged an investigation "as quickly as possible" -- while putting the onus on the U.N. Security Council to "address the need to determine who is responsible and to hold them accountable." 

In 2013, the U.S. and its allies engaged in a stand-off with Syria after Assad was accused of crossing President Obama's "red line" and deploying chemical weapons against the Syrian people. But Assad agreed to relinquish his chemical weapons program under a U.S.-Russian-brokered deal, averting threatened American strikes. 

While inspectors have since chronicled the successful destruction of tons of sarin and other deadly toxic stockpiles, however, chlorine was not part of the U.S.-Russian agreement. 

The Organization for the Prohibition of Chemical Weapons has since concluded "with a high degree of confidence" that chlorine was used on three rebel-held villages in Syria last year, killing 13 people. The OPCW said chlorine had been used "systematically and repeatedly" in Syrian villages and that eyewitnesses claimed that helicopters were used to drop bombs containing chlorine gas. 

The report did not assign blame since it lacks the authority to do so. 

It is widely known, however, that only the Syrian government is using helicopters in the conflict. 

One rescue worker underscored this point to The New York Times, which on Thursday detailed the recent allegations. Regarding witnesses who claimed helicopters dropped chlorine bombs, the worker said: "Nobody has aircraft except the regime." 

The Times reported that attacks, according to rescue workers, have picked up lately in areas like Idlib Province. 

One such attack involving chlorine gas bombs is alleged to have occurred on March 16, killing several people in the town of Sarmin. 

Videos posted online showed people struggling to breathe, and the international humanitarian organization Doctors Without Borders said symptoms described by medics in contact with the group clearly indicate the presence of chlorine poisoning. 

At the time, Secretary of State John Kerry said the U.S. was "deeply disturbed" by the reports and was "looking very closely" at the matter. 

The situation in Syria is even more complicated and dangerous than it was in 2013. Now, the Islamic State has gained ground and the U.S. military is launching airstrikes against the terror network in both Syria and Iraq. Yet, while ISIS opposes Assad, the Obama administration still wants Assad to go -- and is gradually trying to boost moderate rebels in the country. 

Now, as the administration seeks answers on the chemical attacks before the U.N., Russia, Assad's ally on the Security Council, has resisted efforts to place blame on the Syrian regime -- saying there is no proof and sometimes blaming opposition forces for the attacks. Russian Ambassador to the U.N. Vitaly Churkin has sent the U.S. draft resolution to Moscow for review. 

In April, U.S. Ambassador to the U.N. Samantha Power organized for Syrian doctors to present evidence of chlorine attacks to Security Council diplomats. 

The doctors showed a graphic video of medics desperately trying to save children after a chlorine bomb attack on a village in Idlib Province. The images were so horrific that several council members were brought to tears. 

Kerry, in his March written statement on the Sarmin attack, hinted at potential consequences for such attacks. He said a chemical weapons attack through the use of chlorine would be a violation of U.N. Security Resolution 2209, which he said makes clear "such a violation would have consequences." Indeed, that resolution calls for "Chapter VII" action if such weapons are used -- a broad category that can include everything from economic and diplomatic penalties to military action. 

Assad, meanwhile, has denied the charges. In March, he called the accusations "malicious propaganda," and suggested the rebels were behind the latest attack at the time. 

In the interview with CBS News, the Syrian leader also said that he would be open to a dialogue with the United States, but that it must be "based on mutual respect." 

Meanwhile, nobody has yet been held accountable for the attack that led to the 2013 stand-off. 

Fox News' Jonathan Wachtel and's Judson Berger and The Associated Press contributed to this report.