Diplomat: Syria has four chemical weapons facilities it didn't disclose

The diplomat said Sigrid Kaag, the U.N. special envoy overseeing the destruction of Syria's chemical stockpiles, briefed the U.N. Security Council.

Three of the sites are research and development facilities and one is a production facility, according to the source.

Syria disclosed the information during ongoing meetings between the Organization for the Prohibition of Chemical Weapons and Syrian authorities, Kaag told the council members, according to the source.

U.S. Ambassador to the United Nations Samantha Power tweeted shortly after: "Must keep pressure on regime so it doesn't hide (chemical weapons) capability."

In August 2013, U.N. spokesman Martin Nesirky told reporters that a team of experts had gathered to go to Syria to investigate reports of chemical weapons.

In September 2013, U.N. Secretary-General Ban Ki-moon released the inspectors' report, which says there was "clear and convincing evidence" that sarin, a nerve agent developed for chemical warfare, was used in a Damascus attack.

Later that month, the U.N. Security Council passed a resolution requiring Syria to eliminate its chemical weapons arsenal, and President Bashar al-Assad promised to adhere to the resolution. The following month, Syria began dismantling its chemical weapons program.

In July, 600 metric tons of chemical weapons from Syria were transferred to a U.S. ship for destruction in what marked the demise of the last of the country's declared chemical weapons stockpiles.

At the time, officials said it was a major milestone.

But inspectors have said they couldn't say for sure whether some undeclared weapons were still lurking somewhere in the country.

And in August, there were new allegations of chlorine attacks, officials said. After inspectors' findings were detailed in Tuesday's closed-door Security Council briefing, diplomats took to Twitter with statements that echoed accusations aimed at the Syrian government a year ago.

"Witnesses of chlorine gas attacks described them as carried out by helicopters, which only Syrian regime has. All fingers point to Assad," Power said.

"Connect the dots: @OPCW sure chlorine used in #Syria, witnesses saw it dropped by helicopters, which only Assad has. Who's responsible? #UNSC," the Lithuanian mission to the United Nations, a current member of the Security Council, tweeted.

The Syrian mission to the United Nations did not immediately respond to CNN.

Asked about the revelation of additional Syrian chemical weapons facilities, State Department spokeswoman Jen Psaki pointed to a recent statement from Secretary of State John Kerry, which said the United States was "gravely concerned" about the recent reports of chlorine attacks in Syria.

"This finding, coupled with deep concerns regarding the accuracy and completeness of Syria's declaration to the (Organization for the Prohibition of Chemical Weapons), raises especially troubling concerns that continued chemical attacks on the Syrian people by the regime could occur," Kerry said last month.