Measures imposed against the rebel Unita movement in Angola are more effective than they have ever been, according to the head of the United Nations Sanctions Committee for Angola.

But he acknowledged that it would never be possible to completely cut off rebel access to the international diamond market - a main source of finance for Unita leader Jonas Savimbi.

Robert Fowler, who is also Canada's ambassador to the UN, was speaking at the start of a week-long visit to Angola to examine how UN sanctions against Unita might be strengthened.

There is widespread concern that the sanctions, imposed in 1993, have failed to stem Unita's lucrative diamond sales.

In an interview with the BBC, Mr Fowler said the UN could not hope to stop the sale of diamonds completely, but sanctions were making it as difficult as possible.

"It will always be possible for a small parcel of diamonds to reach the international diamond markets.

"Our objective is to make that more difficult and therefore more expensive for Savimbi.

"[We want] to make sure that he is forced to deal with ever more unscrupulous diamond dealers, who take an ever larger margin, leaving him with less money to buy arms."

Sanction busting

The UN Security Council imposed an arms and fuel embargo on Unita in 1993.

But the US group Human Rights Watch says that countries such as Togo, Burkina Faso and elements in Uganda have previously helped break sanctions with flights into rebel territory.

Human Rights Watch also says jets owned by Angola's state oil company, Sonangol, were sighted by UN observers during 1998 in rebel zones, suggesting corrupt government officials have sold fuel to Unita.

Last year, sanctions were expanded to include the ban on Unita's diamond exports, which some estimate have brought in as much as $8bn since 1992.

South African diamond giant De Beers announced last year that it would virtually stop buying Angolan diamonds.

However, the tactic of mixing diamonds from several areas often prevents detection of rebel-harvested gems.

Border anxiety

Despite sanctions, Unita resumed battles with the government in December 1998, shattering a four-year-old UN-brokered peace accord.

The army says it recently found chemical weapons in a Unita arms cache in the central highlands, although it declined to give further details.

Namibian forces have been drawn into the conflict along Angola's southern border as Unita has been pushed to the far south and east of the country.

Unita fighters have crossed into Namibia, and were blamed by the Namibian Government for the recent killing of three French tourists in the north of the country.

Angolan soldiers have entered Namibian territory - with the consent of the authorities in Windhoek - in pursuit of the rebels.

The Angolan army is also reported to have deployed troops along the border with, Zambia, as part of its campaign against Unita