Iran began enriching uranium on Monday to levels unseen since its 2015 nuclear deal with world powers and also seized a South Korean-flagged tanker near the crucial Strait of Hormuz for breaking "environmental laws".

Both decisions appeared aimed at increasing Teheran's leverage in the waning days in office of US President Donald Trump, whose unilateral withdrawal of the US from the nuclear accord in 2018, also known as the Joint Comprehensive Plan of Action, or JCPOA, began a series of escalating incidents.

Increasing enrichment at its underground Fordo facility is pressuring US President-elect Joe Biden to quickly negotiate when he takes over from Trump on Jan 20.

At Fordo, Iranian scientists under the watch of International Atomic Energy Agency inspectors loaded centrifuges with more than 130 kilograms of low-enriched uranium to be spun up to 20 percent, said Kazem Gharibabadi, Iran's permanent representative to the UN atomic agency.

Iran informed the IAEA of its plans to increase enrichment to 20 percent last week. Teheran says its nuclear program is peaceful.

China's Foreign Ministry spokeswoman Hua Chunying said on Tuesday that the Iran nuclear issue was at a critical point in a complicated and sensitive situation.

"We urge all sides to exercise calm and restraint, stick to the commitments of safeguarding the JCPOA, and avoid taking actions that may escalate tensions and leave room for diplomatic efforts and a turnaround in the situation," she said.

Hua said the urgent task for all sides is to push the US to unconditionally return to the agreement as soon as possible and lift all relevant sanctions to create conditions for putting the JCPOA back on the right track.

Iranian state television quoted government spokesman Ali Rabiei as saying that President Hassan Rouhani had given the order to begin the production.

The US State Department criticized Iran's move as a "clear attempt to increase its campaign of nuclear extortion".

Iran's seizure of the MT Hankuk Chemi came as a South Korean diplomat was due to travel to the Islamic republic to discuss the release of billions of dollars in Iranian assets frozen in Seoul.

Iran's paramilitary Revolutionary Guards said on their Sepahnews website that the MT Hankuk Chemi, with 20 crew aboard, was carrying 7,200 tons of oil chemical products.

Iran alleged it seized the vessel over allegations it is polluting the Persian Gulf and the Strait of Hormuz.

Not a tit-for-tat move

Iran's government spokesman Ali Rabiei on Tuesday accused South Korea of holding $7 billion of its funds "hostage" but stressed that Teheran's seizure of the tanker was not a tit-for-tat move.

In past months, Teheran has sought to escalate pressure on Seoul to unlock nearly $7 billion in frozen assets from oil sales earned before the Trump administration tightened sanctions on the country's oil exports.

South Korea has demanded the release of the ship and dispatched its anti-piracy unit to the region.

Agencies, Xinhua and Mo Jingxi in Beijing contributed to this story.