SHANGHAI, China — China today agreed to release sensitive records about missing U.S. soldiers and establish a hot line to the Pentagon, in the latest signs of improving trust between the two militaries.

Details of the agreements, signed at a ceremony in Shanghai, remained hazy, although both have long been sought after by the U.S. military and relatives of thousands of American servicemen missing from the Korean War and other Cold War-era conflicts.

China committed to a military hot line last summer, however, and it was not clear how big a step today’s agreement represented. Calls to spokesmen for the Chinese Defense Ministry and U.S. Embassy defense attache’s rang unanswered.

U.S. officials have said that, at least initially, the arrangement will not give U.S. researchers direct access to Chinese records. Instead, Chinese archivists with security clearances acceptable to the People’s Liberation Army will do the document searches and turn over relevant records to U.S. analysts.

“For the families of the missing, this is extremely significant,” said Charles A. Ray, the deputy assistant secretary of defense for POW-MIA affairs, who participated in the signing ceremony today in Shanghai.

Chinese troops killed and captured thousands of American troops during the Korean War and managed many of the prisoner of war camps set up in North Korea during the war.

More than 8,100 U.S. servicemen are still unaccounted for from the war.

China has periodically cooperated with the Pentagon on matters related to the search for MIAs, but it has consistently maintained that all POW questions were settled at the end of the war.

Declassified U.S. Army records from the 1950s make clear that the United States knew of hundreds of American prisoners in China during the Korean War, closely tracked their movements and feared for their lives.

After visiting China in March 2003 to press for access to military archives, Ray’s predecessor, Jerry D. Jennings, declared, “Chinese records may well hold the key to helping us resolve many of the cases of American POWs and MIAs from the Vietnam War, the Korean War and the Cold War.”

Progress on the U.S.-China military hot line has been slow and last month, the head of the U.S. Pacific Command, Navy Adm. Timothy Keating, complained that the U.S. still doesn’t have a number to call when problems arise.