BEIJING • Most days, the announcement of an $8.5 billion joint venture oil deal between Saudi Arabia and China wouldn't raise eyebrows. Riyadh, after all, is China's largest supplier of oil. But with the United States pushing for an oil embargo against Iran, China's third-largest oil provider, the scenario has changed.

China is emerging as a key test of whether the West and its allies will be able to squeeze Tehran into slowing its nuclear program. The role China will play in the U.S.-led effort is still far from clear, but the United States and others are watching Beijing's moves closely for clues about its intentions.

As the biggest importer of Iranian oil — about 20 percent of Iran's output — China has great sway over American plans. Its decision to accept, partially agree to or spurn the push for new economic curbs on Tehran will be crucial to Washington's efforts to starve Iran's economy as punishment for the country's alleged attempts to develop a nuclear weapon.

For years, and in private, U.S. diplomats have been working quietly to achieve just that outcome. Secret diplomatic cables obtained by WikiLeaks document how the State Department pressed Saudi Arabia to step up its oil exports to China in a bid to weaken rival Iran's foreign oil sales and Tehran's sway over Beijing. It worked, because by 2010, Saudi Arabia had become the top oil supplier to China.

Earlier this month, the European Union and Japan, Iran's second- and third-largest oil customers, signaled willingness to reduce their Iranian oil orders. When and how they do that, however, remains unclear. The EU and Japan combined bought more than 30 percent of Iran's exports in the first half of last year.

The EU announcement was expected to be formalized Monday at a meeting of ministers, but news reports suggest that the embargo could be delayed by six months to allow nations more time to find replacement oil sources.

During a visit last week by Treasury Secretary Tim Geithner, Japan agreed to begin reducing its oil imports from Iran. Soon after Geithner departed, however, Japan began giving conflicting accounts of what it had agreed to do.

China's public actions were the opposite of the EU's and Japan's. Chinese officials were widely seen to have rebuffed efforts by Geithner to get them to sign on during his trip to Beijing last week. China often has criticized American pressure on Iran as unproductive. State-run media slammed U.S. sanctions — announced last Thursday — against a Chinese firm that exports gasoline to Iran.

But Prime Minister Wen Jiabao's presence in Saudi Arabia — a bitter rival of Iran's — spoke otherwise. It raised at least the possibility that Beijing was pondering whether it could offset Iranian oil with supplies from Arab countries should Tehran become a complete financial pariah.