The United States is still mulling ways to stop the fighting in Syria, including a no-fly zone, as the Syrian regime is "ratcheting up the brutality of their tactics," AFP reports citing a US official.
"We continue to look at all of the ideas out there for trying to end the violence," State Department spokeswoman Victoria Nuland said.
Secretary of State Hillary Clinton continues "to talk to partners about how, what, why exactly -- the elements that might go into some of these things that people have proposed, including a no-fly zone," she told journalists.
"But we haven't made any decisions at this stage."
US Acting Under Secretary for Arms Control Rose Gottemoeller met Tuesday with Turkish officials in Ankara to discuss bilateral cooperation on arms control, non-proliferation, disarmament and other security issues.
Her visit came after Turkey forced a Damascus-bound Syrian airliner from Moscow to land in Ankara last week, after receiving intelligence that it was carrying military cargo.
Turkish officials have declined to reveal by whom, or which country, the intelligence had been provided.
UN and Arab League envoy Lakhdar Brahimi, who is visiting Lebanon, has warned that Syria's 19-month conflict could set the entire region ablaze, as President Bashar al-Assad battles to stay in power.
Washington has restricted its aid and support to the Syria rebels to non-lethal help, and has refused calls to arm the opposition.
But reports say weapons are being secretly shipped to the opposition by Saudi Arabia and Qatar, amid some fears that they are ending up in the hands of hardline Islamic rebel groups.
Nuland said the United States was working to ensure that "there be good vetting of who" all the support is going to, including weapons from other countries.
The US was also working with allies to "compare notes on what we are seeing, because it's not just a matter of individual leaders; it's also a matter of ensuring that the groups that are working there are not becoming infiltrated."
She warned the conflict was continuing to escalate.
"We've seen horrific reports of barrel bombs, of cluster munitions used against civilians," Nuland told journalists, but added Washington had not been able to independently verify such reports.
Any no-fly zone over Syria would be implemented for humanitarian reasons to help Syrians displaced inside the country as they seek to escape the conflict that has claimed an estimated 33,000 lives, Syrian rights monitors say.
But experts have warned that setting up such a no-fly zone will be much more complicated than the zone patrolled by NATO in Libya last year.