The US treasury secretary will Tuesday begin a trip to China and Japan to discuss new sanctions on Iran that could raise tensions between Washington and Beijing and pose difficulties for Tokyo.

Both China and resource-poor Japan rely heavily on oil supplies from Iran and are under pressure from the new US measures, which aim to squeeze the Islamic Republic's crucial oil revenues.

The sanctions, signed into law on December 31, aim to increase the pressure on Iran, which Washington accuses of pursuing nuclear weapons -- a claim denied by Tehran.

Timothy Geithner's visit comes a day after the UN atomic watchdog said Iran had begun enriching uranium to up to 20 percent at a new plant in a fortified bunker sunk into a mountain, prompting alarm from Western powers.

The latest sanctions will bar foreign financial institutions that do business with Iran's central bank, which processes most oil purchases in the Islamic Republic, from US financial markets.

Geithner will arrive in China late on Tuesday for the two-day trip, during which he will meet top leaders including Premier Wen Jiabao, before heading on to Tokyo, the US Treasury said.

He will discuss "continued coordination with international partners in the region to increase pressure on the government of Iran, including financial measures targeting the central bank of Iran," it said in a statement.

Energy-hungry China -- which relies on Iran for 11 percent of its imported oil supplies -- has repeatedly said sanctions will not resolve the nuclear issue and has expressed its opposition to the law.

"If you look at it (new law) purely from the letter of the law, it really puts the US and China on a collision course," said Patrick Chovanec, associate professor at Beijing's Tsinghua University.

He said Geithner would use the visit to try to iron out differences with Beijing.

"From a broader strategic perspective, China is very unlikely to accept the US dictating who they can buy oil from and under what conditions," he said.

International sanctions, meanwhile, have left resource-poor Japan searching for alternative supplies, officials say, as Tokyo tries to respond to US and EU concerns over Iran.

Japan is heavily dependent on the Middle East for its energy, with Iranian oil accounting for nearly nine percent of its power needs in the first 11 months of 2011 -- an issue that Geithner is expected to discuss with Japanese leaders.

Geithner will hold talks with Japan's Prime Minister Yoshihiko Noda and Finance Minister Jun Azumi in Tokyo on Thursday.