China "will not protect" whoever sank a South Korean warship in March, Premier Wen Jiabao has said.
"China objects to and condemns any act that destroys the peace and stability of the Korean peninsula," Mr Wen was quoted as saying after talks in Seoul.
South Korea has blamed the North for sinking the Cheonan with a torpedo.
Beijing is under pressure to take a strong stance against North Korea but so far has not accepted the findings of an independent investigation.
"The Chinese government will decide its position by objectively and fairly judging what is right and wrong about the incident while respecting the international probe and responses to it by each nation," said Mr Wen.
Beijing has previously called for all sides to show restraint.
The BBC's John Sudworth in Seoul says Beijing's refusal so far to condemn its old ally has been a major source of frustration to South Korea and President Lee Myung-bak.
But some in South Korea will see Mr Wen's comments as a sign of a subtle and careful shift in position by the Beijing authorities, says our correspondent.
A spokesman for Mr Lee said Seoul was "fully concentrating on diplomatic efforts to hold North Korea responsible" for the 26 March attack on the Cheonan, which left 46 sailors dead.
In a rare news conference on Friday, North Korea again accused the South of "fabricating" evidence, and warned an "extremely serious situation is being created in the Korean peninsula in which a war may break out at any moment".
South Korea says an investigation involving international teams uncovered indisputable evidence that North Korea fired a torpedo at the ship.
Investigators said they had discovered part of the torpedo on the sea floor which carried lettering that matched a North Korean design.
Seoul has announced a package of measures, including a halt to most trade with North Korea and is also seeking action via the United Nations Security Council.
Pyongyang, which fiercely denies the allegations, has retaliated by scrapping an agreement aimed at preventing accidental naval clashes with South Korea.
It also warned of an immediate attack if the South's navy violated the disputed Yellow Sea borderline - the site of deadly naval skirmishes in 1999 and 2002.
On Tuesday, North Korea announced it would sever all ties with the South. It had also banned South Korean ships and planes from its territory.
At the press conference in Pyongyang on Friday, North Korea's policy chief, Maj Gen Pak Rim-su, accused Seoul of trespassing in its waters and "kicking up a frenzy for confrontation".
He said blaming North Korea for the sinking was "an open declaration of war and a heinous criminal act which drives inter-Korean relations to the situation of war and the self-destruction of digging their own grave".
Meanwhile, Japan has said it is tightening its already stringent sanctions against North Korea.
It said it was lowering the amount of cash which individuals can send to North Korea without declaring it from 10m yen ($110,000; £76,000) to 3m yen.
The parliament in Tokyo also passed a bill to enable the Japanese coastguard to inspect vessels on the high seas suspected of carrying North Korean weapons or nuclear technology, in line with a 2009 UN Security Council resolution.
The Associated Press news agency quoted the head of the Public Security Intelligence Agency as saying he had ordered officials to keep a closer eye on the some one million North Koreans living in Japan.
North and South Korea are technically still at war after the Korean conflict ended without a peace treaty in 1953.