The United States believes that Syria's large stock of chemical weapons will be destroyed on schedule by the end of June 2014, US officials said Thursday.

"I am increasingly confident that we will be able to complete this task, the elimination of the Syria's chemical weapons program, within the target date of June 30th of next year," said Thomas Countryman, assistant secretary for international security and non-proliferation.

He was speaking as the UN's chemical watchdog said Syria's entire declared stockpile had been placed under a seal and all its chemical arms production equipment destroyed in line with a November 1 deadline.

The work is being carried out under an ambitious UN resolution, cobbled together by the United States and Russia, to eliminate Syrian President Bashar al-Assad's entire chemical weapons arsenal.

"Our target dates are ambitious but they are achievable. We have the support of the international community," Countryman told US lawmakers at a hearing about the conflict in Syria.

The United States said it wanted to "applaud the OPCW, the United Nations and the Joint Mission staff for their unprecedented work" on eliminating Syria's chemical weapons.

State Department spokeswoman Jen Psaki added that "rendering this equipment inoperable is an important step in ensuring that chemical weapons are never used again by the Assad regime against the Syrian people."

US ambassador to Syria Robert Ford told lawmakers that "the destruction of the regime's chemical weapons is a huge success if in fact it is carried out fully."

"All stocks of chemical weapons and agents have been placed under seals that are impossible to break," Organization for the Prohibition of Chemical Weapons spokesman Christian Chartier told AFP, adding the seals were "tamper proof."

"These are 1,000 tonnes of chemical agents (which can be used to make weapons) and 290 tonnes of chemical weapons," Chartier said in The Hague.

OPCW inspectors had until Friday to visit all of Syria's chemical sites and destroy all production and filling equipment in line with a timeline laid down by the Hague-based OPCW and backed by the UN Security Council resolution passed last month.

The Executive Council of the Nobel peace prize-winning organization will now meet again on November 5 to decide by November 15 on "destruction milestones" for the stockpile.

The UN resolution was agreed by the US and Russia to avert military strikes on Syria after deadly chemical weapons attacks outside Damascus in August, which the West blamed on Assad's regime.

But Ford acknowledged in the Senate committee hearing that opposition rebels had reacted with "anguish" to President Barack Obama's decision not to press ahead with military strikes.

"They're deeply disappointed, Senator, that we chose not to use military force. I have heard -- just anguish from people that I have talked to over there," Ford said.

He added however that the US had delivered ten pick-up trucks to the Syrian opposition military forces, as part of a US commitment to help them in their fight against Assad.

More than 350,000 ready-to-eat rations as well as three tons of medical equipment has also been delivered to rebel forces, Psaki said.

Ford, who was forced to leave Damascus in 2011 because of the fighting, has been working hard behind the scenes to unify the fractured Syrian opposition and bring them to the negotiating table to chart a political transition in Syria.