Image caption China is the North's closest ally

Chinese Prime Minister Wen Jiabao is visiting South Korea amid continuing tension over the sinking - allegedly by North Korea - of a South Korean warship in March.

The issue is expected to dominate Mr Wen's talks on Friday with the South Korean President, Lee Myung-bak.

International investigators have blamed North Korea for sinking the warship.

But China has so far refused to join in the widespread condemnation of its old ally, Pyongyang.

The Chinese state news agency Xinhua quoted the Chinese ambassador to South Korea, Zhang Xinsen, as saying the visit would "strongly boost" ties between the two countries.

The BBC's correspondent in Seoul, John Sudworth, says however, that China is in an uncomfortable position.

Frustration

He says China's refusal, so far, to join the strong, international condemnation of North Korea is causing frustration in Seoul.

ANALYSIS China has too much at stake to rush into any position on the sinking of the South Korean warship. There is a chance that any instability in the region could lead to tens of thousands of North Korean refugees streaming across the long border that separates China and the North. North Korea also acts as a buffer between China and the US-backed South. It serves Beijing's strategic interests to prop up Pyongyang's current government. China has also got its hands full managing its own developing society - the last thing it wants is trouble on its doorstep. So China's leaders might be privately furious at North - if indeed it did sink the Cheonan - but it will think long and hard before criticising its leaders.

There are calls for China to come off the fence and choose between its old, cold war alliance with Pyongyang and its new role as a modern, global power.

Some news reports suggest that Mr Wen will, albeit subtly and carefully, move closer to the South Korean position on this visit, choosing a form of words that at least placates some of the anger.

But, our correspondent adds, China's priority is stability on its borders and it will be reluctant to sign up to anything it thinks will provoke its North Korean neighbour.

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: £75,000) 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.

SINKING OF CHEONAN - KEY DATES March 26: Explosion hits naval corvette near disputed maritime border, killing 46 on board

May 20: Independent investigators produce proof North Korean torpedo struck vessel

May 24: South Korea declares trade with North frozen, demands apology

May 25: North Korea announces it is severing all ties with South Korean propaganda fight Korean War armistice Q&A: Cheonan sinking Timeline: North Korean attacks

Since being accused of the torpedo strike on the South Korean ship, North Korea has said it will scrap 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.

On Tuesday, North Korea announced it would sever all ties with the South.

The Yellow Sea was the site of deadly naval skirmishes in 1999 and 2002.

The two states are technically still at war after the Korean conflict ended without a peace treaty in 1953.