VIENNA, Austria — Russia is increasing pressure on Iran to be more open about its nuclear program, threatening to indefinitely withhold fuel for a Russian-built reactor unless Tehran lifts secrecy shrouding past nuclear activities, diplomats said yesterday.

Russia warned in March that it would not provide fuel rods for the reactor it is building in the southern city of Bushehr as long as Iran ignored U.N. Security Council demands that it freeze uranium enrichment, diplomats said.

Now, Moscow has modified that demand, saying no fuel will be provided unless Iran meets another key international request — that it fully explain past activities that heightened suspicions it might be looking to develop a nuclear arms program, two diplomats familiar with Iran’s nuclear file told the Associated Press.

The two, commenting separately, spoke on condition of anonymity because their information was confidential.

The International Atomic Energy Agency, as well as Iranian and Russian officials, declined comment.

Moscow has played a complicated role in attempts to pressure Tehran to comply with international demands.