Iran and world powers have resumed talks in Vienna aimed at bringing the U.S. back into the nuclear deal it left in 2018.

Following a positive round of talks last week, the mood feels very different in the Austrian capital after an attack on a major Iranian nuclear facility on Sunday.

Iran blames Israel for the incident in Natanz and has said it will increase its uranium enrichment to 60 percent in response.

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That is up from around 20 percent at the moment and far higher than the 3.67 percent stipulated in the 2015 nuclear deal, known as the Joint Comprehensive Plan of Action, or JCPOA.

Israel has neither confirmed nor denied the allegations about the attack on Natanz.

Iran's Foreign Minister Javad Zarif tweeted as talks got under way on Thursday: "Iran's seriousness of purpose in pursuing diplomacy was tested in the three years since Trump withdrew from the nuclear accord. Iran – by remaining in the deal – passed with flying colors.

"The Biden administration, however, has only shown a commitment to Trump's maximum pressure."

White House press secretary Jen Psaki said "the U.S. was not involved in any manner" in the Natanz attack.

U.S. President Joe Biden has vowed to renegotiate the nuclear accord, from which his predecessor Donald Trump withdrew in 2018 as he said it was too skewed in Tehran's favor.

The U.S. and Iran are not meeting directly in Vienna, with the other signatories to the deal – the UK, France, Germany, China and Russia – and diplomats from the EU serving as mediators shuttling between the two sides.

On Wednesday, France, the UK and Germany released a joint statement calling Tehran's decision to ramp up uranium enrichment "regrettable" and said they noted the move with "grave concern."

But, Iran insists it's within its rights to retaliate following the attack and called the European response "weak."

Speaking after the latest talks got under way, China's representative called for more urgency to secure a new agreement and prevent further escalation.

"Currently, we should do away with all disruptive factors, moving forward as swiftly as we can on the work of negotiation, especially by zeroing in on sanctions-lifting," said Wang Qun, Chinese ambassador to the International Atomic Energy Agency.

Doing so would require Iran and the U.S. to come closer together, at a time when the political rhetoric suggests they're heading in opposite directions.