Suspension of a United Nations update on casualties “will be a loss — we will now have only disparate sources of information,” said Hamit Dardagan, an author of a report on Syrian casualties by the Oxford Research Group, a London-based organization that put the toll at 113,700 as of November.

Mr. Dardagan, a founder of Iraq Body Count, a project begun in 2003 to record civilian casualties from the war in Iraq, also said the sectarian nature of the Syrian crisis would further complicate any data collection. “As any conflict intensifies and you have more refugee flows and more people displaced, that becomes more difficult,” he said in an interview.

The number of nongovernmental organizations able to work in Syria has been reduced by the increasing violence. Civilian groups that report events considered unfavorable to any of the warring parties have been targeted; most recently, Razan Zeitouneh, a rights activist who ran the Violations Documentation Center, and her colleagues were abducted from their office in a Damascus suburb. Parties on both sides also actively filter information they provide to the outside world to help their cause, and government restrictions and the threat of kidnapping and death have severely limited access for journalists.

United Nations agencies that do have some access have also described problems in verifying data. Officials with the World Food Program and the World Health Organization in Damascus said recently that because their officials could not reach many areas in Syria, they had set up local contacts to relay information to them, but that verification was difficult. Government ministries provide some data, but they are out of touch with branches in some rebel-held areas.

The United Nations reported some progress on Tuesday in the international effort to purge Syria’s chemical weapons stockpile. In a statement issued with the Organization for the Prohibition of Chemical Weapons, it said the first batch of the most dangerous materials in the stockpile had been exported from the country, loaded onto a Danish commercial vessel in the Syrian port of Latakia.

The statement said the Danish vessel would remain at sea until the second cargo of chemicals reached Latakia, when it would return to load them. The vessel was escorted by Danish and Norwegian naval vessels, the statement said, and China and Russia were providing further maritime security for the operation.

“This movement initiates the process of transfer of chemical materials from the Syrian Arab Republic to locations outside its territory for destruction,” said the statement by Sigrid Kaag, the United Nations official responsible for coordinating the effort.