U.N. Secretary General Ban Ki-moon smiles before delivering a speech during the opening ceremony of the United Nations Industrial Development Organization General Conference in Lima on Monday. (ENRIQUE CASTRO-MENDIVIL/REUTERS)

U.N. Secretary General Ban Ki-moon has expressed grave concern about the safety of international inspectors overseeing the destruction and removal of Syria’s chemical weapons, warning that a surge in fighting among Syrian combatants poses a risk to their lives.

Ban voiced his concerns in a letter to the U.N. Security Council that provides fresh details on international plans for the elimination of Syria’s arsenal. A copy of the Nov. 27 letter, had not been made public, was posted Tuesday on the Web site of a reporter from the Arabic-language broadcaster Al Hurra and later authenticated by diplomats.

The joint mission, made up of 15 experts from the Organization for the Prohibition of Chemical Weapons (OPCW) and 48 U.N. personnel, is preparing the ground for the latest and riskiest phase of the operation: transporting large quantities of chemical agents through a war zone to the Syrian port of Latakia, where they will be shipped outside the country.

Diplomats are hopeful that international negotiators will agree to accept an offer by the United States to destroy the chemical agents at sea, perhaps after they have been transferred there by Norwegian and Danish vessels.

Ban said the United Nations has received assurances from the warring parties to cooperate in the transport of chemical agents. Nevertheless, he said, “recent fighting in the Syrian Arab Republic shows that the security situation is volatile, unpredictable and highly dangerous.”

Ban voiced particular concern about the safety of the mission’s main headquarters in Damascus, saying the United Nations is installing “security enhancements.” All armored U.N. vehicles, he noted, have been equipped with communications and tracking systems, and staff have received extra security training. “Despite these measures, the facility remains vulnerable to certain risks, and the joint mission is actively exploring viable alternative locations to base its activities, should the security situation require it,” he wrote.

In September, Secretary of State John F. Kerry and Russian Foreign Minister Sergei Lavrov reached agreement on a framework to destroy Syria’s chemical weapons by the middle next year.

The pact — which averted a U.S. airstrike against Syria in retaliation for its alleged use of chemical weapons against its own people — has largely gone smoothly, with the OPCW confirming Oct. 31 that Syria completed “the functional destruction of critical equipment for all of its declared chemical weapons production facilities and mixing/filling plants, rendering them inoperable.”

But the latest phase — which calls for the destruction of chemical weapons materials outside Syria — has been dogged by setbacks. Several countries that had been asked to oversee the destruction of the chemicals on their soil, including Norway and Albania, have refused. The United States has since agreed to destroy the materials at sea.

Ban’s safety concerns come as the United Nations’ chief humanitarian relief coordinator, Valerie Amos, told the Security Council behind closed doors Tuesday that living conditions are sharply deteriorating in Syria.

The country is facing one of the world’s worst humanitarian crises in decades, with more than 9 million civilians in need of assistance, and more than 2.5 million people largely cut off from aid. Nearly 250,000 civilians are living under a state of siege, mostly at the hands of government forces, facing the threat of starvation.

Diplomats said Amos told the 15-nation council that the Syrian government has vowed to lift a few bureaucratic hurdles that have hindered the U.N. relief effort in Syria, pledging to grant 50 new visas to relief workers. But, speaking to reporters after the Security Council briefing, Amos said, “On some of the more difficult areas — protection of civilians, demilitarization of schools and hospitals, access to besieged communities and also cross-line access to hard-to-reach areas — we have not seen any progress.”

Last week, Syria pledged for the first time in the conflict to allow the United Nations to run aid convoys from Jordan, Iraq and Lebanon.

Amos said that Syria had also acceded to a long-standing U.N. request to open humanitarian hubs in three towns, Aleppo, Suwayda and Qamishli, in order to deliver aid to hard-to-reach communities. But she said Syria has refused to permit goods to enter through southern Turkey, a conduit for the rebels’ military supplies but also one of the most-concentrated areas of civilian humanitarian need. “They see crossing the Turkish border as a red line,” Amos said.