US concerned as Syrian regime moves helicopters close to rebel stronghold

The build-up of regime forces around Idlib is believed by the US to be a signal that an all-out assault on the enclave could begin within days, several US officials tell CNN.

Officials stress they are also worried that an assault on the city using conventional weapons could leave thousands dead and deepen the humanitarian crisis along the Syria-Turkey border. They believe chemical weapons could be used if the rebels are able to slow regime advances.

The potential for another round of chemical attacks has set off a diplomatic row with Russia.

"I will assure you that the Department of State has been in active communication, recent active communication with Russia to enlist them in preventing this, now I'll just leave it at that for right now, but the communication is going on," Defense Secretary James Mattis told reporters Tuesday.

The State Department has also shared concerns about "any kind of escalation" of violence in Idlib with the Russian government, according to State Department spokesperson Heather Nauert.

"We have shared the concerns that we have about any potential offensive taking place, we have shared those concerns with the Russian government at many levels," she said Wednesday.

There is also a sense that Russia is seeking to sow doubt if and when a chemical attack takes place. "That is their mode" one US defense official said. Early indications of the Russian approach emerged Wednesday when Russian Foreign Minister Sergey Lavrov said he was concerned about "staged provocations" by rebel groups that would be blamed on the Syrian government. He also called remaining Syrian rebel groups in Idlib "terrorists," and that an all-out assault on the city is likely.

On a more direct military level, Russia has claimed the US is building up a Navy presence in the eastern Mediterranean to attack Assad. Pentagon spokesman Eric Pahon said, "What I can tell you is that Russian reports of a US military buildup in the Eastern Med are nothing more than propaganda. It's not true. That does not mean, however, that we are unprepared to respond should the President direct such an action."

US officials say in turn Russia has significantly increased its level presence in the eastern Mediterranean off Syria. NATO has weighed in as well.

"The Russian Navy has dispatched substantial naval capabilities to the Mediterranean, including several ships equipped with modern cruise missiles," said NATO chief spokeswoman, Oana Lungescu. "We will not speculate on the intention of the Russian fleet, but it is important that all actors in the region exercise restraint and refrain from worsening an already disastrous humanitarian situation in Syria.