The claim by Human Rights Watch (HRW) assumes significance in light of an escalating controversy over 380 tonnes of explosives that the International Atomic Energy Agency (IAEA) says went missing from Iraq after the US invasion.
"In May 2003, Human Rights Watch provided US and British forces with specific data, including precise ... coordinates on unsecured weapons stockpiles around Baghdad and in Basra," the New York-based group said in a statement on Thursday.
However, "coalition forces took little or no action to secure the stockpiles", it added.
"Immediately after the fall of Baghdad, our researchers were finding massive stockpiles of weapons and explosives throughout Iraq," executive director of HRW Kenneth Roth said.
US forces reportedly failed to
secure the weapons' sites
The group said its researchers had found a massive stockpile of warheads, anti-tank mines, anti-personnel mines and other weapons at the unsecured Second Military College on the main road between Baghdad and Baquba.
"The weapons stocks were in the process of being looted," Roth said.
"But US coalition forces did not move to secure the site."
The statement came as US officials scurried to explain reports that 380 tonnes of explosives had gone missing from a warehouse in Iraq.
UN nuclear watchdog IAEA fears that the explosives might have fallen into the hands of terrorists.
The missing explosives have become a major issue in the US presidential elections, with Democratic challenger John Kerry accusing President George Bush of messing up the war.