Azerbaijan released a shipment of Russian equipment for Iran's first nuclear power plant on Thursday, more than a month after it was halted at the border, a customs official said.

The cargo passed through the Astara customs checkpoint on the border with Iran, said the official with Azerbaijan's State Customs Committee. He spoke on condition of anonymity since he was not authorized to speak to the media.

Azerbaijan had halted the cargo of heat-isolating equipment headed for the Bushehr plant on March 29, and demanded more information from Russia about the nature of the material. Azerbaijani officials said they feared the equipment could violate United Nations sanctions.

The Russian state-run company building Bushehr, OAO Atomstroiexport, accused Azerbaijan of deliberately obstructing the cargo.

Atomstroiexport officials could not immediately be reached for comment Thursday.

Iran is paying Russia more than $1 billion to build the light-water reactor — Iran's first atomic energy plant. Construction has been held up by disputes between Tehran and Moscow over payments and a schedule for shipping nuclear fuel.

The United States and other Western nations that fear Iran is seeking to acquire nuclear weapons have criticized Russia in the past for building Bushehr.

Washington, however, softened its position after Iran agreed to return spent nuclear fuel to Russia to ensure it does not extract plutonium from it that could be used to make atomic bombs.

The United States and its Western allies also agreed to drop any reference to Bushehr in the sanctions resolutions passed by the U.N. Security Council as a result of Russian pressure.

Russia says the plant's contract is in line with all international agreements aimed at preventing nuclear weapons proliferation.