By Jonathon Burch
ANKARA (Reuters) - Turkey said on Friday that any use of chemical weapons by Syrian President Bashar al-Assad would "take the crisis to another level", but remained cautious about any foreign military intervention in the conflict on its border.
The White House said on Thursday Assad's government had probably used chemical arms on a small scale, but that President Barack Obama needed proof before he would act.
"We have been hearing allegations of the use of chemical weapons for quite some time now and these new findings take things to another level. They are very alarming," Turkish foreign ministry spokesman Levent Gumrukcu said.
"Since the very first reports of chemical weapons being used in Syria emerged, we have been asking for a thorough investigation by the United Nations to substantiate these reports. However, the Syrian regime has not allowed this."
Syria, which has so far denied access to U.N. investigators because of a dispute over their remit, denies firing chemical weapons and accuses anti-Assad rebels of using them.
"This has been done by organisations, including al Qaeda, which threatened to use chemical weapons against Syria. They have carried out their threat near Aleppo. There were victims," Syrian Information Minister Omran al-Zoubi said in Moscow.
"The Syrian army does not have chemical weapons," Interfax news agency quoted Zoubi as saying.
He compared the U.N. mission with the U.N. inspectors sent to Iraq in the 1990s to check for weapons of mass destruction that former leader Saddam Hussein was suspected of accumulating.
A decade ago, U.S. President George W. Bush used inaccurate intelligence to justify an invasion of Iraq to destroy nuclear, chemical and biological weapons that turned out not to exist.
"Somebody is very keen to send an investigative commission to Syria similar to the one that used to work in Iraq and in the end led to its occupation and destruction," Zoubi said.
Russian Foreign Ministry spokesman Alexander Lukashevich echoed that assessment, saying plans to look into other alleged incidents of chemical weapons use in addition to the attack near Aleppo were "like the scheme used ... in Iraq".
In a statement, he suggested Western nations in the U.N. Security Council "were concerned not with concrete steps to prevent the use of toxic substances in the Syrian crisis but with the goal of changing the regime of a sovereign state".
Syria's uprising is the bloodiest and longest of Arab revolts that erupted more than two years ago. It began with peaceful protests against Assad that were met with force, sparking armed opposition and eventually a full-scale civil war.
More than 70,000 people have been killed. A military stalemate has set in, but Assad has still been able to rely on support from Russia and Iran.
A once-fervent advocate of foreign intervention in Syria, Turkey has grown increasingly frustrated with the fractured opposition to Assad and with international disunity.
Asked whether Turkey would allow foreign military action in Syria from its soil, Gumrukcu said the facts about chemical weapons usage needed to be substantiated first.
"Let's not jump to that right now. Let's have a thorough investigation," he said, adding that any response if the claims were verified would need to be discussed among the "Friends of Syria" grouping of the opposition's Western, Arab and other allies.
The U.S. disclosure created a quandary for Obama, who has set the use of chemical weapons as a "red line" that Assad must not cross. It triggered calls from some hawkish Washington lawmakers for a U.S. military response, which the president has resisted.
On Friday, Obama said deployment of chemical weapons by Syria's government was a "game changer", but also noted that intelligence assessments proving they had been used were still preliminary.
Turkey had been pushing for a foreign-protected "safe zone" inside Syria that could serve as a refuge for civilians caught up in the chaos and ease the burden on refugee camps in Turkey, now housing more than a quarter of a million people.
But it has been less vocal in recent months and officials were privately cautious about the latest U.S. disclosure.
"(The) statements are very vague and they themselves do not seem to be very confident of their arguments," one source close to the Turkish government said.
"Turkey has been voicing some concerns to that end as well, but without proof, I don't think any further steps than the current level of involvement would be made," the source said.
"Intervention is very risky."
The European Union also responded cautiously, saying it hoped the United Nations would be able to send its investigating mission to Syria to check for chemical weapons use.
"We are still monitoring this along with our international partners to see what has really happened because it doesn't seem entirely clear at this point in time," said Michael Mann, a spokesman for EU foreign policy chief Catherine Ashton.
"We've seen that the regime in Syria doesn't seem to have much respect for human life, but we can't be definitive on this until we see definitive evidence," Mann said.
(Additional reporting by Nick Tattersall in Istanbul, Adrian Croft in Brussels, Gabriela Baczynska in Moscow and Jeff Mason in Washington; Writing by Nick Tattersall; Editing by Alistair Lyon and Tom Pfeiffer)