WASHINGTON (Reuters) - The Syrian government may be developing new types of chemical weapons, and U.S. President Donald Trump is prepared to consider further military action if necessary to deter chemical attacks, senior U.S. officials said on Thursday.

FILE PHOTO: A U.N. chemical weapons expert, wearing a gas mask, holds a plastic bag containing samples from one of the sites of an alleged chemical weapons attack in the Ain Tarma neighbourhood of Damascus, Syria August 29, 2013. REUTERS/Mohamed Abdullah/File Photo

President Bashar al-Assad is believed to have secretly kept part of Syria’s chemical weapons stockpile despite a U.S.-Russian deal under which Damascus was supposed to have handed over all such weapons for destruction in 2014, the officials said.

Assad’s forces have instead “evolved” their chemical weapons and made continued occasional use of them in smaller amounts since a deadly attack last April that drew a U.S. missile strike on a Syrian air base, the officials told reporters in a briefing.

Characteristics of some of those recent attacks suggest that Syria may be developing new weapons and methods for delivering poison chemicals, possibly to make it harder to trace their origin, the officials said, speaking on condition of anonymity, but they declined to provide specifics.

A deadly sarin attack on a rebel-held area in April prompted Trump to order a missile strike last year on the Shayrat air base, from which the Syrian operation is said to have been launched.

“We reserve the right to use military force to prevent or deter the use of chemical weapons,” one official said, while declining to specify how serious a chemical attack would have to be to draw a fresh U.S. military response.

A second official said, however, that the Trump administration hopes that stepped-up international sanctions and diplomatic pressure will help rein in Assad’s chemical weapons program.

If the international community does not act quickly to tighten the screws on Assad, Syria’s chemical weapons could spread beyond its borders and possibly even “to U.S. shores,” the second official said.

“It will spread if we don’t do something,” the official warned.

The officials echoed U.S. Secretary of State Rex Tillerson’s recent accusation that Russia, Assad’s ally in Syria’s multi-sided civil war, bears some responsibility for failing to enforce the chemical weapons ban.

Russia has denied any complicity, and the Syrian government has said it has not carried out any of the attacks.

The U.S. officials suggested that if left unchecked there would be more smaller chemical attacks as an “instrument of terror” to compensate for Assad’s lack of adequate manpower to retake some opposition-held areas.

“They think they can get away with it if they keep it under a certain level,” an official said.

Western officials have cast suspicion on the Syrian government for a chlorine gas attack on a rebel-held enclave east of Damascus last week that sickened at least 13 people.

State Department spokeswoman Heather Nauert said on Thursday the United States was “extremely concerned” about reports that Syrian forces had carried out another chlorine gas attack this week in the eastern Ghouta area.

The U.S. officials also said Islamic State militants have sporadically used chemical weapons such as sulfur mustard and chlorine, some via improvised explosive devices, a more rudimentary tactic than those of the Syrian government.