Syria is sharply criticizing an upcoming event on the U.N. investigative body that is assisting in documenting serious crimes committed during the seven-year Syrian conflict, including possible war crimes and crimes against humanity.

Syria's U.N. ambassador, Bashar Ja'afari, said in a letter to General Assembly President Miroslav Lajcak circulated Friday that the Sept. 27 event is another "destructive" action by its sponsors, Qatar and Liechtenstein.

The event will take place on the sidelines of the 193-member assembly's annual gathering of world leaders.

The U.N. missions of Qatar and Liechtenstein did not immediately respond to requests for comment.

Ja'afari reiterated Syria's claim that the investigative body established by the General Assembly in December 2016 is illegal and violates the U.N. Charter. He said any evidence it collects "will be ineligible for future criminal proceedings."

The resolution adopted by the assembly said the body, known as the "International, Impartial and Independent Mechanism," would help collect and analyze evidence of violations of human rights and international humanitarian law "to facilitate and expedite fair and independent criminal proceedings."

It called for the body "to closely coordinate" with the Independent International Commission of Inquiry on Syria, which was established by the Geneva-based U.N. Human Rights Council. The commission has alleged that Syrian government and rebel forces committed war crimes and it accused the government of using chemical weapons.

When the resolution was adopted, Ja'afari called the resolution "a flagrant interference in the affairs of a U.N. member state." He said it was "a direct threat to a solution" to the Syrian conflict, which has killed at least 400,000 people, according to monitoring groups.

In the letter circulated Friday, Ja'afari said the situation in Syria "is now at a delicate stage" and the political process led by U.N. special envoy Staffan de Mistura "is moving forward, but with caution and fragility."

This is because "some governments that have supported chaos and terrorism in Syria" are pressuring the government and its allies "in their war on global terrorism," he said.

Ja'afari accused the U.N. of abandoning its neutrality and responding to "political and financial pressure and the polarization practices of some member states," especially those supporting the investigative body.

Syria "has the full capability, with its legal and judicial institutions, to achieve justice and accountability without external and destructive interference," he said.