Visiting UN food agency officials on Wednesday stressed the need for major food aid for North Korea, while a cautious South Korea called for transparent distribution of any assistance.

The seven World Food Programme delegates led by Terri Toyota, director of government donor relations, met senior officials of the South's unification ministry which handles cross-border affairs.

Toyota briefed them on the agency's recent visit to the communist North but did not officially appeal for food aid from the South, the ministry said in a statement.

"Toyota said (the North) needs 434,000 tonnes in aid for 6.1 million people in the most vulnerable population such as children, the elderly and pregnant women," it said.

The ministry said it would "make reviews in consideration of overall inter-Korea relations... and monitoring to ensure transparent distribution".

The figures are the same as those given in a report last week by the WFP and the UN's Food and Agriculture Organisation, following their assessment visit to the North.

The report said more than six million people urgently need assistance because of substantial falls in domestic production, in food imports and in overseas aid.

The North suffered famine in the 1990s which killed hundreds of thousands and has relied partly on international food aid ever since.

But donations to UN programmes have dwindled due to international irritation at the North's missile and nuclear programmes.

The North has reportedly asked some 40 countries for food aid, notably the United States whose programme was suspended in 2009 over a monitoring row.

South Korea used to supply its neighbour with an annual 400,000 tonnes of rice but this ended in 2008 as relations soured.

Ties have worsened further since the North's alleged sinking of a Seoul warship in March 2010 that killed 46 sailors, and its bombardment of a border island last November that killed four.

The bombardment led Seoul to suspend shipments of humanitarian aid by private charity groups.

Some Seoul officials say last year's harvest was relatively good, and Pyongyang may be trying to stockpile food for celebrations next year of the 100th anniversary of the birth of founding president Kim Il-Sung.

China has reinforced fences and patrols along parts of the border with North Korea in an apparent bid to ward off a growing stream of refugees from its impoverished neighbour, a report said.

Four-metre-high (13.2 foot) fences topped with barbed wire are being erected along the Yalu River around the Chinese city of Dandong facing the North's northwest city of Sinuiju, Yonhap news agency reported late Tuesday.

The river, which marks the border in the North's northwest, is a popular escape route for those seeking to flee permanently or to find food in China.

Previously the border was only marked by a three-metre-high fence which "anybody could cross if they really wanted", said a Dandong resident quoted by Yonhap in its report from the city.

Work on the new fence sections, which currently stretch for about 13 kilometres (8 miles), began last November and is continuing, Yonhap said. New patrol posts were being built on higher ground to improve monitoring.

"It's the first time such strong border fences are being erected here. Looks like it's related to the unstable situation in North Korea," the resident was quoted as saying.

A growing number of North Koreans have fled the country amid continuing food shortages. Almost all cross first to China, which repatriates those whom it catches as economic migrants.

Refugees often travel on to third countries before eventual resettlement in South Korea.

More than 20,000 North Koreans have arrived in the South since the end of the 1950-1953 war, the vast majority in recent years.