As a primary backer of the Syrian government, Iran has argued vehemently against US airstrikes, warned that sectarian "fire" will spread, and that jihadi rebels may have been behind the Aug. 21 chemical weapons attack that US officials say killed more than 1,400 people near Damascus.

According to leaked diplomatic correspondence, Iran has been warning Washington since July 2012 that Sunni rebel fighters have acquired chemical weapons, and called on the US to send “an immediate and serious warning” to rebel groups not to use them.

In a letter acquired by The Christian Science Monitor that was sent sometime in the spring, Iran told American officials that, as a "supporter" of the rebels, the US would be held responsible for any rebel use of chemical weapons.

Iran amplified those year-old warnings over the weekend in its strongest public comments to date linking the rebels with a chemical weapons, echoing Russia's dismissal of American assurances that President Bashar al-Assad's forces were to blame. The comments come as the US Congress prepares to vote on military strikes.

“There is ample intelligence that takfiri [extremist] groups are in possession of chemical arms,” Iran’s Foreign Minister Mohammad Javad Zarif said on Sunday during a visit to Iraq, according to state-run PressTV. “Extremists and takfiris are a threat to the whole region.” Semi-official Fars News Agency headlined its story: “Iranian FM refutes US claims on Syria’s use of chemical weapons.”

Multiple warnings

Iran says that it warned the United States directly, in mid- and late- 2012, and at least once after that, about the risks of chemical weapons among the rebels. The letter acquired by the Monitor references messages from July 18 and Dec. 1, 2012.

According to the English translation that accompanies the one-page Persian document, the letter reads: “Alerting [worrying] news has been published about the preparations of insurgent forces in Syria for using chemical weapons/elements."

Iran “holds responsible, in addition to the elements of violent forces, their supporter countries including the American government, for any resort to chemical weapons/elements by those insurgent forces,” it states.

The letter makes no reference to the possibility of chemical weapons use by Syria itself – holder of the world’s third-largest chemical arsenal. Nor does it acknowledge that if the same argument was applied to the regime, then Iran and Russia, Syria's closest supporters, would likewise be held responsible for any regime use of chemical weapons.

The Iranian letter is undated and was produced by the previous government under President Mahmoud Ahmadinejad, according to informed sources in Iran who provided a copy to the Monitor on the condition that they not be further identified.

A US State Department official would not verify the authenticity of the letter as a matter of policy, stating: “We don’t comment on diplomatic correspondence.”

Neither the Persian and English versions of the letter seen by the Monitor have any official letterhead or other identifying marks, but that is not unheard of with sensitive Iranian communications. The grand bargain offer sent by Tehran to the White House in 2003, for example, looked similarly “clean” – authentic and with approval from Iran’s highest authorities, yet with no sign of that on the document itself.

Consistent opposition

Since the Aug. 21 attack, Iranian officials have repeatedly stated their opposition to chemical weapons use by any side – a policy that has been consistent since Iran was targeted by chemical munitions repeated in the Iran-Iraq War of the 1980s. But they has sent mixed messages about who they believe was responsible for this attack on the rebel-held suburbs of Damascus.

While making the case on Sept. 7 for punitive US strikes on Syria, Samantha Power, the US Ambassador to the United Nations, said that the US “cannot afford to signal to North Korea and Iran that the international community is unwilling to act to prevent proliferation or willing to tolerate the use of weapons of mass destruction.”

Yet Ms. Power added, however, that Iran and the Russians had complied with American requests and sent messages to Syria “to try to dissuade Assad from using chemical weapons” – a known red line laid down by President Barack Obama. As reports of chemical incidents have grown inside Syria, so too has information that Iran and Russia have advised Mr. Assad not to use such weapons, in a conflict that has already taken more than 100,000 lives by conventional arms.

Mr. Zarif first revealed that Iran had sent direct warnings to the US via the Swiss Embassy in Tehran in an interview published Sept. 1 by Aseman weekly in Tehran. The Swiss have handled US interests in Iran throughout the 34-year US-Iran estrangement, and have been a conduit for such messages in the past.

Zarif said in the interview that Iran sent a memo to the US last December stating that “handmade articles of chemical weapons, including sarin gas, are being transferred into Syria.” He added: “In the same note, we warned [Washington] that radical groups might be planning to use these chemical agents.”

Zarif said the US never responded to the letter.

The document seen by the Monitor appears to be a third letter, which recalls the “messages” of the two previous letters, but does not mention any specific details about the types of chemical weapons that might be in rebel hands, or where they came from. Nor does it mention what information, intelligence, or "published" news may have prompted the Iranians to write it.

Instead, it refers to two well-known incidents of chemical weapons use by Iraqi forces, the mustard gas attack on the Iranian town of Sardasht in 1987, and the gassing of the Iraqi Kurdish town of Halabja in 1988 – both of which the letter says Iran holds “Western countries” accountable for by backing Saddam Hussein in the war, and drawing a parallel with Western-backed rebel forces today.

Conflicting claims

Reports of chemical weapons incidents began to multiply at the beginning of 2013, with accusations leveled at both sides but always denied. At the end of May, Turkish media reported the arrest of seven members of the rebel-allied jihadist group Jabhat al-Nusra in two southern Turkish cities, as well as the discovery of two kilograms of sarin gas in the suspects' homes.

Speaking today in London, US Secretary of State John Kerry said that the Aug. 21 attack which targeted 11 rebel zones and emanated from government-controlled areas could only have been conducted by regime forces, and and not rebels who do not have the "scientific capacity” to mount such a sophisticated chemical attack.

The Iranian letter is written from a perspective of a Syria supporter, with what one Iranian analyst who has seen the text notes is the “rough tone” that characterized Mr. Ahmadinejad’s descriptions of the US and Western actions in the region.

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The letter states: “Precedence and recent experiences of Syrian crises shows that, the violent forces [groups], emboldened by assurances of comprehensive political and military support given by some countries including the US, have so far applied no limitation in violence or brutal crimes against innocent people.”

The letter adds that Iran “will not spare any effort” to find a “peaceful solution.” Iran “would like to urge the US government to prevent any undermining [of] the non-proliferation of WMDs” and avoid “a potential human tragedy” by “sending an immediate and serious warning to the insurgent forces in Syria about any resort to chemical weapons/elements.”