Iran has continued disputed nuclear work despite Western calls to halt
Iran has not expanded the number of centrifuges enriching uranium at its nuclear facilities since the end of May, diplomats have told the BBC.
But they said there had been an increase in the number of centrifuges installed and that they could be brought online within weeks.
It is not clear whether the slowdown is due to technical or political reasons.
Centrifuges spin uranium hexafluoride gas to produce enriched uranium, which can be used to make nuclear weapons.
Iran has been given until September by the UN Security Council to end its uranium enrichment programme or face tougher sanctions.
The West suspects the country is secretly trying to develop nuclear arms, while Tehran insists its programme is entirely peaceful.
For the past three years, Iran has steadily increased its stock of enriched uranium by using centrifuges at its Natanz nuclear facility, south of Tehran, to spin uranium hexafluoride gas and increase the proportion of fissile uranium-235 atoms.
For uranium to work in a nuclear reactor it must be enriched to contain 2-3% uranium-235. Weapons-grade uranium must contain 90% or more u-235.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
In its latest report on Iran's nuclear programme in June the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said that nearly 5,000 centrifuges had been enriching uranium by 31 May.
On Tuesday, diplomats said the number had fallen slightly in recent weeks because some had been taken down for repair and maintenance.
However, they cautioned that the number of centrifuges that had been installed but had not yet become operational had risen from around 2,100 in May.
These could be added to production lines within a few weeks, if desired, said the Washington's Institute for Science and International Security (Isis).
The diplomats said it seemed unlikely the Iranian government had deliberately chosen to curb enrichment, and that the reason was more likely technical.
There has so far been no immediate comment from Iranian officials.
Isis recently estimated that even if Iran continued using only the 5,000 operational centrifuges, it could accumulate enough enriched uranium to produce two nuclear warheads by February 2010.
The BBC's Bethany Bell in Vienna says the IAEA is expected to issue its latest report on Iran in the next few days.
Its findings could influence talks among world powers considering the possibility of harsher sanctions on Tehran over its nuclear work, our correspondent says.