LONDON — British spies during World War II were frustrated by the lack of information-sharing with the FBI and feared Nazi agents could infiltrate Britain through America, newly declassified documents show.

Files released yesterday by the National Archives chart the rocky early years of the relationship between the American and British intelligence agencies and show how cooperation improved over the course of the war. The files on trans-Atlantic relations are part of a package of documents from the domestic spy agency MI5 — also known as the Security Service — released by the National Archives. The documents show that in 1941, before America had entered the war, MI5 officers were arguing for closer intelligence cooperation with the American agency. They feared German agents could hide themselves among the thousands of American diplomats, military personnel, journalists, and businesspeople entering the country in the wake of the Lend-Lease agreement under which America agreed to supply material support to the Allies.

“The 30,000 Americans who are arriving over here and the many hundreds here already, who at the moment are subject to little control, represent a grave danger to security, and it is advocated that the FBI should send their own representative to cooperate with the British Security Authorities,” one official, P.E. Ramsbotham, wrote. Elsewhere, the same officer notes that “Americans are notoriously indiscreet and often find difficulty in resisting the blandishments of journalists in search for copy.”

Mistrust existed on both sides. During a 1942 visit to America, senior MI5 officer Guy Liddell was told by a Canadian official that the FBI “think you are cagey and that you do not trust them. You ask them to make enquiries, but you do not give them the full facts of the case.”

The British, in turn, felt there was an “inadequate supply of information about Axis espionage cases in the United States.”