BEIRUT Both sides in Syria's conflict on Wednesday demanded an international inquiry into a deadly attack they each cite as evidence that the other has used chemical weapons.
The deaths of 26 people in a rocket attack on a northern town on Tuesday have become the focus of a propaganda war between President Bashar al-Assad's supporters and opponents, who accuse each other of firing a missile laden with chemicals.
The United States and Russia, which back opposing sides in Syria, took contrasting views of the strike on Khan al-Assal, near Aleppo, which, if confirmed, would be the first use of chemical weapons in the two-year-old conflict and could step up pressure for foreign military intervention.
Robert Ford, the U.S. ambassador to Damascus who left Syria more than a year ago, said his government had no evidence so far to substantiate reports that chemical weapons munitions had been used in Syria on Tuesday.
"But I want to underline that we are looking very carefully at these reports," he told a House of Representatives hearing.
Ford warned of unspecified consequences for Assad's government if it were found to be using chemical weapons. Washington, which says Assad must be removed from power, has so far shunned direct military intervention in Syria.
Russia initially backed the Syrian government's version, issuing a statement on Tuesday, expressing extreme concern about "the use by the armed opposition of chemical weapons".
But on Wednesday, Moscow's line had softened.
"There is no unequivocal evidence on this account yet," Deputy Foreign Minister Gennady Gatilov said on Twitter.
Syria's restrictions on independent media and foreign aid groups make it hard to verify what happened, but survivors interviewed in hospitals complained of breathing difficulties.
The opposition Syrian National Coalition said it wanted an international investigation into the alleged chemical attack in Khan al-Assal and on another in Otaiba, a town near Damascus.
"The Coalition would like all parties and individuals involved in this reprehensible crime to be brought to justice," it said in a statement.
Syria asked U.N. Secretary-General Ban Ki-moon to set up an independent technical team "to investigate the use of chemical weapons by the terrorists in Khan al-Assal", state TV reported.
Rebels might latch onto any suspicions of chemical weapons use to bolster a case for Western military intervention, although not all of Assad's opponents favour this.
Britain, which along with France wants the European Union to relax its arms embargo on Syria to help rebels, said reports of the chemical attack strengthened their case.
"The French president is concerned and I am concerned that we should not be restricted for months and months ahead when we don't know exactly what could happen in Syria, including the very worrying reports of use of chemical weapons," British Prime Minister David Cameron told parliament.
Iran, Assad's closest ally, summoned the Swiss ambassador - who represents U.S. interests in the Islamic Republic - to voice suspicions that the United States might be responsible if the rebels obtain chemical weapons.
"(Iran) emphasises the American government's responsibility in preventing rebels from acquiring and using chemical weapons," Foreign Ministry official Mohsen Naziri told IRNA news agency.
The World Health Organization said it would send medical supplies to Aleppo but could not verify if chemical agents had been used. Experts contracted by the U.N. agency will visit Aleppo health facilities to identify immediate health needs and provide support for treatment of toxins, it said.
Syria's military has turned increasingly heavy weapons on Assad's opponents, using firearms, then tanks, artillery, aircraft and long-range missiles, but it has denied it would use chemical weapons, if it had them, on its own people.
David Friedman, a former specialist on weapons of mass destruction with Israel's Defence Ministry, said he did not believe chemical weapons had been used in the Khan al-Assal attack, citing witness testimony and television footage.
Some of those hospitalised after the attack told a Reuters photographer of a strong smell of chlorine in the air and that many victims had fallen down dead after the blast.
Friedman said a rocket might have hit chemicals stored in the area. "As for the smell of chlorine, this could have been due to the indirect explosion of something containing industrial chemicals," he argued, saying this was common in urban combat.
At least 70,000 people have been killed by conventional weapons in Syria's two-year-old conflict, but conflicting claims about chemical weapons are often bandied about.
The United States and Israel fret that Syria's presumed chemical arsenal might fall into the hands of Islamist militants fighting Assad, or be diverted to his Lebanese Shi'ite Hezbollah allies - fierce foes of the Jewish state.
The Syrian state news agency SANA said the government had sent two letters to the United Nations in the past voicing fears that hostile powers may supply chemical weapons to "terrorists" to fabricate claims that the government is using such arms.
The Information Ministry has also said al Qaeda-linked Nusra Front rebels had previously seized a private factory in eastern Aleppo that had tonnes of poisonous chlorine material. (Additional reporting by Dominic Evans and Oliver Holmes in Beirut, Dan Williams in Jerusalem, Zahra Hosseinian in Zurich and Steve Gutterman in Moscow; Editing by Alistair Lyon and Robin Pomeroy)