(CNN) -- The United States warned Syria on Monday not to use chemical weapons amid intelligence reports indicating President Bashar al-Assad's regime could be preparing to take that step as it escalates its fight against rebel forces.
"I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people," Secretary of State Hillary Clinton said Monday. "But suffice it to say, we are certainly planning to take action if that eventuality were to occur."
The Syrian Foreign Ministry denied that the country had any plans to use chemical weapons, state TV reported. But U.S. intelligence officials say "worrying signs" suggest otherwise.
"This isn't just about movement, but about potential intent to make certain chemical weapons ready for use," an intelligence official told CNN on condition of anonymity because of the sensitivity of the matter.
The official declined to describe the intelligence and acknowledged that the United States isn't entirely sure what the Syrian government is up to, or who ordered the moves.
President Barack Obama has warned that any use of chemical weapons by Syria in its civil war would be crossing a "red line" that would prompt a swift U.S. response.
Meanwhile, the United Nations said Monday that it is immediately pulling all nonessential employees out of Syria. It was unclear how many people were involved or what role they have been playing in Syria. The United Nations removed cease-fire monitors from the country in August.
On Monday, at least 90 people died across Syria, including 10 killed when Syrian warplanes bombed a town within sight of the Turkish border.
Thick black smoke rose from the border town of Ras al-Ain, where witnesses said the warplanes dropped two bombs. One appeared to strike a three-story building where opposition forces were staying, neighborhood mayor Mehmet Saitavci said.
The strike sent panicked civilians running to the fence that separates the two countries, witnesses told CNN.
Saitavci said the wounded were making their way to the border, where they were being picked up by ambulances.
"There are people with arms and legs missing coming across," he said.
In Damascus, apparent fighting around the airport forced Egypt's national airline to cancel flights to Syria, including recalling one flight that had taken off, after Syrian authorities contacted the airline to say the security situation was bad "at the airport and its vicinity," airline spokesman Dina el-Fouly said.
The airport had been closed for three days because of fierce fighting, and Egypt Air had planned to resume flights Monday. They are now canceled indefinitely, el-Fouly said.
Elsewhere, government forces bombed, shelled and rained rocket fire on cities across the country in the latest efforts by al-Assad's forces to drive back rebel advances, opposition activists said.
The airstrikes signal a sharp escalation in the fighting by forces loyal to al-Assad and rebels seeking his ouster, raising concerns among Syria's neighbors that the 21-month-old civil war could spill across the borders.
Neighboring countries have reported deadly border skirmishes with either Syrian forces or rebels.
In June, Syrian anti-aircraft defenses shot down a Turkish military reconnaissance jet, killing two pilots, after it briefly crossed into Syrian airspace in the eastern Mediterranean Sea. Months later, errant Syrian artillery shells hit the border town of Akcakale, killing five Turkish civilians.
As a result, Turkey has asked NATO for Patriot missiles to bolster its air defenses, a request NATO is expected to approve by Tuesday.
The United States, Germany and the Netherlands, which all have Patriot capabilities, have signaled they may be willing to contribute missiles should NATO approve the deployment to Turkey.
However, Russia reiterated its opposition.
"We don't consider that this will lead to the improvement of security in the current situation." President Vladimir Putin's spokesman, Dimitri Pesvok, said Monday.