Rwanda has warned it is ready to send troops back into the Democratic Republic of Congo (DRC) in response to fighting along the border.

Rwandan forces backing Congolese Rally for Democracy (RCD) rebels pulled out of the DRC only this month after years of involvement in the civil war there.

If they attack Bukavu we shall react

Anastase Gasana Rwandan ambassador to the UN

But the country's ambassador to the United Nations, Anastase Gasana, said on Friday that it would not stand by and let forces loyal to the DRC government "exploit" the withdrawal.

The United Nations food agency has warned that a humanitarian catastrophe is imminent in the conflict area as a result of the fighting between the RCD and the pro-government Mai-Mai militia.

The Mai-Mai seized the town of Uvira earlier in the week and are now threatening Bukavu, about 120 kilometres (70 miles) to the north, which borders the Rwandan town of Cyangugu.

"If they attack Bukavu we shall react. We shall not wait for them to take it," Mr Gasana told the French news agency AFP on Friday.

He said that troops withdrawn from the DRC had not been demobilised and were "on the alert".

The RCD rebels have warned the Mai-Mai they have 48 hours to leave Uvira or be "flushed out".

'Evidence'

The Rwandan ambassador accused DRC President Joseph Kabila of directing the Mai-Mai attack - a charge which Kinshasa denies.

Crowds in Bukavu watch the Rwandans go earlier this month

Rwandan President Paul Kagame earlier told the BBC he had concrete evidence that Kinshasa was providing the Mai-Mai with arms and logistic support - a charge Kinshasa denies.

Mr Kagame said such support was a serious breach of the agreement under which Rwandan troops withdrew from the eastern DRC.

Rwanda says that most of the Mai-Mai fighters who seized Uvira are actually former Rwandan soldiers and Interahamwe militiamen, responsible for the 1994 genocide.

Food shortage

Bukavu is still reported to be tense after earlier warnings of an assault from the Mai-Mai.

Many shops are reported to be closed in the town and UN workers there say they have their bags packed and are ready to leave.

World Food Programme spokesman Jean-Charles Dei says about 1.3 million people are now in need of food aid in the region of the conflict.

This represents an increase of 500,000 since the fighting began 10 days ago, following the Rwandan withdrawal.

The WFP spokesman said immediate action was needed to avert a disaster.