THE HAGUE: The world's chemical weapons watchdog is pressing Syria to explain why it has four undeclared warfare agents, its head said Wednesday, after a U.S. official accused Damascus of continuing to hoard a toxic stockpile.

Organization for the Prohibition of Chemical Weapons chief Ahmet Uzumcu said despite previous declarations by Syria, OPCW teams have found indications of five additional chemical agents.

After recent consultations with The Hague-based OPCW's secretariat, Syria "declared research and development of one more chemical agent," Uzumcu said in a report released last week, of which AFP was given a copy on Wednesday.

But "at present, Syria has not yet adequately explained the presence of indicators of four chemical warfare agents," Uzumcu said.

The OPCW chief added that "new information" offered by Damascus has failed to resolve outstanding issues on Syria's chemical warfare program.

"In many instances, such new information presents a considerable change in narrative ... from previous information – or raises new questions," Uzumcu said.

Uzumucu said the OPCW's secretariat believed if Syria's effort continued "without a change in approach" its declaration "is unlikely to yield concrete results."

It's been almost three years since a U.S.-Russian brokered deal in September 2013 saw Syria cave in to international pressure to hand over its chemical stockpile to the OPCW for destruction.

Syria's admission comes after a sarin gas in August that year on rebel-held areas near Damascus that was blamed by the West and the opposition on Syrian President Bashar Assad's government.

The removal of the weapons was the result of the historic deal that averted threatened U.S. airstrikes against Damascus after the August attacks.

- Syria 'cynically' exploiting pact -

But on Tuesday the U.S. permanent representative to the OPCW voiced frustration with Syria's perceived lack of cooperation in the process to verify its chemical arsenal.

Kenneth Ward said the OPCW's latest findings were "indicative of (the) production, weaponization and storage of chemical warfare agent by the Syrian military."

This "has never been acknowledged by the Syrian government," Ward said in an address at OPCW, obtained by AFP on Wednesday.

"We therefore remain very concerned that chemical warfare agent and associated munitions, subject to declaration and destruction, have been illicitly retained by Syria," he said.

In January, the OPCW announced that all Syria's declared chemical arms had been completely destroyed, despite concerns that sarin gas and other chemical weapons were still being unleashed in the country's complex civil war that has so far killed more than 280,000 people.

Damascus has furiously denied ever using chemical arms and instead said the accusations "only served political agendas".

But Ward, in a strongly-worded statement, said there was a "body of evidence indicating that Syria never truly accepted the obligations or ideals of the Chemical Warfare Convention."

"For more than two years, the (OPCW's) Secretariat and Council provided Syria with an opportunity to instill international confidence that it had renounced chemical weapons," said Ward.

"Syria has not only squandered that opportunity, it has cynically exploited it," the U.S. representative said.