TEHRAN, Iran -- Iran has informed the UN nuclear watchdog that it will increase its nuclear enrichment capacity within the limits set by the 2015 agreement with world powers.

The modest steps announced Tuesday appeared to be mainly aimed at signalling that Iran could resume its drive toward industrial-scale enrichment if the nuclear accord comes unraveled.

Behrouz Kamalvandi, the spokesman for Iran's nuclear agency, was quoted by state TV as saying a letter was submitted to the International Atomic Energy Agency detailing the move.

Kamalvandi said Iran is "providing infrastructure and arrangements for high-speed and capacity in production of UF4 and UF6 gases as well as rotor of centrifuges."

Spinning centrifuges convert the gases into enriched uranium that can be used for reactor fuel and medical isotopes. If enriched to higher levels, the material can be used for weapons.

The head of Iran's nuclear agency, Ali Akbar Salehi, said Iran is prepared to resume work on advanced centrifuges that would dramatically increase its capacity for enrichment. But he said that so far the work is limited to building a new facility for assembling the centrifuges.

Supreme Leader Ayatollah Ali Khamenei had ordered the increase in capacity in a speech Monday, in which he vowed that Iran would preserve its nuclear program despite the U.S. withdrawal from the landmark 2015 accord.

The agreement set strict limits on Iran's uranium enrichment in return for the lifting of U.S. and international sanctions.

When the Trump administration withdrew from the accord, it announced that it would resume sanctions and impose new penalties unless Iran dramatically changes its policies on other issues not covered by the deal, including its ballistic missile program and support for regional militant groups.

Iran currently is using nearly 5,000 centrifuges and enriching uranium at 3.5 per cent. It says it needs more enriched uranium for its only nuclear power plant. Iran denies it has ever sought nuclear weapons, which require uranium enriched to 90 per cent.