By Jill McGivering
As Washington, now joined by the EU3, presses for punitive international action against Tehran, one of its most difficult tasks will be to win China's support.
China needs energy to fuel its economic boom
The first step is to persuade China to agree to support - or not to block - an International Atomic Energy Agency (IAEA) referral of Iran to the UN Security Council.
China, like Russia, feels it is in an awkward position.
An important development for Beijing will be how Russia decides to react.
Initial reports from Washington, soon after news broke that Iran was resuming work, said that Moscow had privately agreed not to veto referral to the Security Council but it is still unclear if that is true.
If it were, Beijing could find itself diplomatically isolated. That would only increase the pressure on Beijing to follow Moscow's lead.
But Beijing would like to avoid that crisis altogether if it possibly can. Its own focus is firmly on a non-confrontational diplomatic solution to the crisis.
Beijing's initial reaction to news that Iran was breaking its deal with the EU3 was to express its concern, but immediately reaffirm its commitment to multilateral negotiations.
Since then, the diplomatic temperature has increased dramatically but China has refused to change its position.
Officials have repeated the Chinese government's view that the best way forward is to restart the EU3 diplomacy with Iran, despite the fact many in the West are now dismissing it as exhausted.
China's work behind the scenes seems to be focussed on trying to keep the diplomacy alive.
China's most obvious interest is energy.
Energy supply is a priority for the Chinese government
Three years ago, when Iran was already supplying 13 per cent of China's oil needs, the two governments signed a major deal which included Chinese development of Iranian oil fields.
It is a source of supply of growing importance for China - one it doesn't want disrupted by politics.
China also has a deeply-engrained reluctance to takes sides with the US against a fellow non-Western nation.
Much of its current energy-driven diplomacy is on forging political alliances which exclude the West and are faithful to Chinese principles of non-interference in each other's internal affairs.
But Beijing is also keen not to cause fresh tensions in its relationship with Washington.
Compliance on Iran may be seen by Washington as an important test of its sincerity.
'Force for peace'
The Bush administration is pressing China hard to be a more engaged and responsible player on the international stage as it emerges as an increasingly dominant world power.
Support on North Korea and Iran are exactly what it has in mind, a way of proving to Washington that China is, as it claims, a force for peace in the world and can be trusted at a time of crisis.
China has shown itself willing to play an active role as long as the focus in both cases is on peaceful diplomacy but it's unclear whether China would be prepared to endorse US-led punitive action which could be detrimental to its own interest.
Chinese willingness to take sides with the US against a friend and energy supplier like Iran could alarm some of its other suppliers, from Sudan to Burma.
All of this will be high on the agenda of Hu Jintao's forthcoming visit to Washington, expected in April.
For China, they are impossible choices. As Beijing scours the world for oil and gas, its strategy is to keep politics and energy as separate as possible, however impossible a task that is starting to look.