International inspectors overseeing the destruction of Syria’s chemical weapons stockpile have missed an early deadline in a brutally tight schedule after security concerns prevented them from visiting two sites linked to Damascus’ chemical program.

Experts from the Organization for the Prohibition of Chemical Weapons were to have checked all 23 of Syria’s declared chemical sites by Sunday, but the organization said Monday that inspectors have visited only 21 because of security issues. While there are no consequences for missing the deadline, the group’s failure to meet it underscores the ambitious timeline as well as the risks its inspectors face in carrying out their mission in the middle of Syria’s civil war.

The OPCW did not say who was responsible for the security problems, but the organizations’ director-general has said in the past that temporary cease-fires may have to be negotiated between rebels and forces loyal to President Bashar Assad to reach some sites. The chemical weapons watchdog said it has not given up hope of gaining access to the two locations.

“Negotiations continue to try to get security guarantees so our inspectors can go in,” OPCW spokesman Michael Luhan said.

The joint OPCW-U.N. mission faces a string of target dates for specific tasks as it aims to achieve the overall goal of ridding Syria of its chemical stockpile by mid-2014. Luhan said the next deadline is Nov. 1, by which time Syria has to complete “functional destruction of the critical equipment for all its chemical weapons production facilities and mixing-filing plants.”

That step will ensure that Syria can no longer make new chemical weapons. After that, the international community and Syria have to agree to a plan to destroy the country’s chemical stockpile.

Syria is believed to possess around 1,000 metric tons of chemical weapons, including mustard gas and the nerve agent sarin. It has sent the OPCW a plan for full destruction of the stockpile that has to be discussed by the group’s executive council next month.

The OPCW, based in The Hague, said such declarations by member states “provide the basis on which plans are devised for a systematic, total and verified destruction of declared chemical weapons and production facilities.”

State Department spokeswoman Jen Psaki said Monday that Washington was reviewing the declaration, which ran to more than 700 pages. “We are, in accordance with OPCW regulations, not going to publicly discuss or analyze our assessment of the report,” she said.

The two sites the inspectors still need to check appear to be in rebel-held or contested areas. At least one of the locations is believed to be the town of al-Safira, which experts say is home to a production facility as well as storage sites. The area has been engulfed by fighting for months, and many of the rebels in the area are from al-Qaida-linked groups.

The OPCW-U.N. mission stems from a deadly chemical attack on rebel-held suburbs of Damascus in August that killed hundreds. Assad denied any role in the attack, while the U.S. and its allies blamed his government and threatened to carry out punitive missile strikes.

The U.S. and Russia then brokered an agreement for Syria to relinquish its chemical arsenal. Assad quickly agreed, and the deal was enshrined in a U.N. Security Council resolution.