North Korean leader Kim Jong-Il visited a factory in northeastern China on Friday as he travelled through the nation which is his cash-strapped country's chief ally, local residents said.

Kim, who travels aboard an armoured train, is in China after holding his first summit with Russian President Dmitry Medvedev in Siberia, where he offered a nuclear concession that was greeted with suspicion by Washington.

The North Korean leader is known for shunning air travel and taking extraordinary security measures, and residents in the Chinese city of Qiqihar where he spent the night said there was a heavy police presence on the streets.

"I couldn't say how many there were, but it seemed as though the city's entire police force were on the streets," one local resident told AFP by telephone on condition of anonymity.

"He has just left, and the police have gone."

A woman who answered the phone at a machinery factory in the city, in northeastern China's Heilongjiang province, said Kim spent around half an hour visiting the plant and speaking to officials.

The source, who did not give her name, said Kim left at around 10:00 am (0200 GMT).

Beijing has declined to say whether Kim is on an official visit, or just passing through on his way back from Russia.

Previous visits by Kim to China have been shrouded in secrecy and only officially confirmed by either side after his departure.

State news agency Xinhua said Kim crossed into China by train at the border city of Manzhouli on Thursday.

It is his first visit since May, when he told China he remained committed to the denuclearisation of the Korean peninsula, and follows a Kremlin announcement that North Korea was ready to resume dialogue without preconditions.

The United States and South Korea -- who along with China and Japan make up the other six countries involved in the process -- dismissed the proposal as nothing new.

One Chinese user of the Twitter-like service Weibo who called himself Liu Bohui posted a message online reporting that Kim Jong Il's visit to Qiqihar had caused traffic chaos.

"The whole of Qiqihar is a traffic jam," Liu wrote. "I'm stopped in the road. This is the biggest traffic in Qiqihar I've ever been caught in."