LONDON (Reuters) - Iran has reopened a nuclear plant idle for nine years, its atomic energy agency (AEOI) said on Wednesday, as Tehran prepares to increase uranium enrichment capacity if a nuclear deal with world powers falls apart after the U.S. withdrawal.

U.S.-Iranian tensions have resurged since President Donald Trump pulled Washington out of the 2015 nuclear accord, calling it deeply flawed. Under the deal, Iran restricted its enrichment program to ease concerns it could not be put to developing nuclear weapons and in return won relief from sanctions.

European signatories are trying to save the accord, which they see as crucial to forestalling an Iranian nuclear weapon. However, Iranian Supreme Leader Ayatollah Ali Khamenei ordered the AEOI this month to start preparations to upgrade enrichment capacity in case the European efforts fail.

The AEOI said on Wednesday that in response to Khamenei’s order and Trump’s renunciation of the deal, a plant for the production of UF6, the feedstock for centrifuge machines that enrich uranium, had been relaunched and a barrel of yellow cake has been delivered there.

Uranium ore, known as yellow cake, is converted into a gas called uranium hexafluoride (UF6) before enrichment.

The UF6 factory, which had been inactive since 2009 due to a lack of yellow cake, is part of the Isfahan uranium conversion facility, according to AEOI’s statement on its website.

“Iran has imported a huge amount of yellow cake since the nuclear deal” in 2015, and had also produced some domestically.

The International Atomic Energy Agency, the U.N. nuclear watchdog that is policing Iran’s compliance with the nuclear deal, said on June 5 that the AEOI had informed it of “tentative” plans to resume production of UF6.

The move is symbolic and permissible under the nuclear deal, which allows Iran to enrich uranium to 3.67 percent, far below the 90 percent of weapons-grade uranium, and caps its stock of enriched uranium hexafluoride at 300 kilograms (660 pounds).

President Hassan Rouhani has written to counterparts in France, Germany and Britain, warning that time to salvage the nuclear deal is running out.

Rouhani’s chief of staff, Mahmoud Vaezi, was quoted as saying on Wednesday by the government’s website that Rouhani had expressed Iran’s demands “very clearly” in this letter.

Washington will start reimposing some economic penalties on Tehran in August and more in November.

The tightening of U.S. sanctions pressure has set Washington and Tehran, adversaries since Iran’s 1979 Islamic Revolution, back on a course of confrontation after a period of cautious detente under Trump’s predecessor Barack Obama. Rouhani urged Iranians on Wednesday to “bring America to its knees”.