International weapons inspectors announced Monday that Syria has handed over the last of its declared chemical weapons stockpile for removal and destruction, even as U.S. officials voiced concern about the Damascus government’s alleged use of other toxic substances.

The Organization for the Prohibition of Chemical Weapons (OPCW), which was charged last year with overseeing the destruction of Syria’s chemical weapons under an agreement between the United States and Russia, took control of the “last consignment” of the deadly chemicals, which were loaded onto a ship at the Syrian port of Latakia, the agency’s director general, Ahmet Uzumcu, said in a statement.

“A major landmark in this mission has been reached today,” Uzumcu said Monday. “The last of the remaining chemicals identified for removal from Syria were loaded this afternoon aboard the Danish ship Ark Futura.”

He said the next step involves delivering the chemicals “for destruction at the assigned facility on board the U.S. vessel Cape Ray and at commercial facilities” in the United States, Britain, Finland and Germany.

Secretary of State John F. Kerry hailed the OPCW’s announcement, but he also pointed to what he called alarming evidence that the Syrian government has begun using chlorine gas in rudimentary bombs dropped from aircraft on opposition residential areas.

“It’s very important, however, even as we mark this moment of removing 100 percent of the declared weapons, that we understand that our work is not finished,” Kerry told reporters during a diplomatic visit to Iraq.

“There are still some serious issues that remain to be addressed, and we are not going to stop until those have been addressed,” Kerry said.

The Syrian government has delayed and obstructed the OPCW in its work, and there are still questions about whether Syria’s initial declaration of its stockpiles was complete, he said.

In a news conference in The Hague, Uzumcu said the last shipment amounted to 8 percent of Syria’s 1,300-ton stockpile.

The Syrian government agreed to surrender its chemical arsenal last fall when President Obama threatened to launch missile strikes in retaliation for deadly chemical attacks on rebel-held suburbs of the Syrian capital, Damascus. U.S. officials said the attacks in August killed more than 1,400 civilians, including at least 426 children. A U.S. government assessment concluded that the attacks were the work of the Syrian government of President Bashar al-Assad.

Uzumcu described the chemical disarmament program that resulted from the U.S.-Russian agreement as unprecedented.

“Never before has an entire arsenal of a category of weapons of mass destruction been removed from a country experiencing a state of internal armed conflict,” he said. “And this has been accomplished within very demanding and tight time frames.”

Uzumcu added that although a major chapter in OPCW’s efforts is closing, the organization’s work in Syria will continue. “We hope to conclude soon the clarification of certain aspects of the Syrian declaration and commence the destruction of certain structures that were used as chemical weapons production facilities,” he said.

The disarmament program ground to a halt in April because Syria insisted on holding on to 27 tons of sarin precursor chemicals as leverage in a dispute with the international community over the future of facilities used to store the deadly agents, U.S. officials said. The Assad government missed an April 27 deadline to turn over the remaining 8 percent of its chemical arsenal, which was stored in 16 containers in Damascus, the officials said.

In a written statement expanding on his remarks Monday at a news conference in Baghdad, Kerry said, “In the coming weeks, the United States stands ready to begin destruction of a large amount of Syria’s chemical weapons precursors.” But he stressed that “very serious issues remain and must be resolved.”

In addition to the reported use of chlorine gas, he said, “the Syrian regime has dragged their feet on destroying production facilities,” and the world continues to be “appalled at the death toll from the regime’s continued attacks on its own people, as well the grave humanitarian crisis.”

“The worst of the weapons are gone, but the despicable regime and the crisis it has created remain and require our collective focus,” Kerry said.

Anne Gearan in Baghdad contributed to this report.