The United States and Uzbekistan have quietly negotiated and are expected to sign a bilateral agreement today to provide American aid in dismantling and decontaminating one of the former Soviet Union's largest chemical weapons testing facilities, according to Defense Department and Uzbek officials.

Earlier this year, the Pentagon informed Congress that it intends to spend up to $6 million under its Cooperative Threat Reduction program to demilitarize the so-called Chemical Research Institute, in Nukus, Uzbekistan. Soviet defectors and American officials say the Nukus plant was the major research and testing site for a new class of secret, highly lethal chemical weapons called ''Novichok,'' which in Russian means ''new guy.''

The agreement to help Uzbekistan clean up the plant is part of wide-ranging cooperation between Tashkent and Washington since the former Soviet republic of Uzbekistan became independent in 1991. Yesterday, American and Uzbek officials opened a series of meetings in Tashkent, the Uzbek capital.

Uzbek officials said in interviews earlier this year that, only after their country became independent, did they come to understand the legacy of pollution that had resulted from their designated role as the Soviet Union's major testing ground for chemical and biological weapons. ''We were shocked when we first learned the real picture,'' said Isan M. Mustafoev, the Deputy Foreign Minister, in an interview in Tashkent last March.