International Atomic Energy Agency officials are expected to press Iran for access to the Parchin military site near Tehran during their two days of talks. Iran has not yet granted U.N. requests to visit the facility, where officials suspect Iran has built a container for explosives tests.
Western diplomats say they suspect Iran is buying time to remove any incriminating evidence before they allow U.N. inspectors inside the facility.
Talks between Tehran and the five permanent members of the U.N. Security Council plus Germany are set for May 23 in Baghdad. The six world powers, known as the P5+1, are seeking to assess the possible military capability of Iran's nuclear sites.
The Washington Post quotes unnamed U.S. officials Monday saying Iran has been routinely switching off satellite tracking systems on its sea-bound oil tankers since early April. The newspaper also reports that the Iranian practice - a violation maritime law - is being closely monitored by the International Energy Agency.
Switching off the tracking system has only been modestly effective in hiding the massive tankers.
Iran relies on oil exports for the majority of its foreign currency earnings. U.S. officials say the sanctions are drastically hurting Iran's economy, and the value of Iran's currency has dropped sharply.
Western powers suspect Iran is developing nuclear weapons under cover of a civilian energy program, a charge Tehran denies.
Iran and the six world powers resumed talks on the dispute last month in Istanbul after a more than year-long stalemate.
Some information for this report was provided by AFP and Reuters.