South Korea’s JoongAng Daily reports:
According to one source, Chinese diplomats based in Seoul have been telling diplomats from other countries that taking the Cheonan case to the Security Council was “not a good idea” because it could “upset” the North.
South Korea has not decided whether to raise the issue at the Security Council, though that has been mentioned as a possible countermeasure. But already, the South has engaged in diplomatic efforts seeking international help.
China is a crucial player in this picture. It is one of five permanent members on the Security Council with veto power. Any binding resolution at the Security Council requires a unanimous vote, and South Korea fears China – North Korea’s biggest ally and benefactor – may exercise its veto power.
And China has stopped short of condemning North Korea for the attack. Without commenting on the North’s actions, China’s Foreign Ministry called for “calm and restraint” Thursday and said it supports peace and stability in the region.
For more on the China-Iran-U.S. triangle, see “US mum over China’s links to Iran” from Asia Times:
China and the United States have been down a rocky road together over the past two decades with respect to China’s missile technology transfers to Iran. Today, China’s ongoing contributions to the buildup of Iran’s missile forces warrant closer scrutiny.
The opening by Iran of a new missile production plant in March will enable Iran to further quickly expand its supply of Nasr anti-ship missiles. Although no Chinese officials attended the opening ceremony, there are Chinese footprints all around this facility. 
In addition, Iran is preparing to launch several satellites. As in the case of North Korea, each of these Iranian satellite launches will generate its own shockwave in the West, and will spark further debate about the inability of the US and its allies to deal effectively with Iran and its significant technological advances.