The Russian offer comprises two water-dropping planes and two helicopters, he said.

"Negotiations between Portugal and Russia are underway," the official said on condition of anonymity.

Prime Minister Jose Manuel Durao Barroso's government last week asked the North Atlantic Treaty Organisation (NATO) to provide up to six Canadair water-dropping aircraft along with three heavy-duty helicopters and crews.

The Portuguese request was channelled through a NATO emergency centre which coordinates aid for member states to combat natural disasters.

Germany, Morocco and Spain have also offered aircraft to help Portugal fight the fires, which on Tuesday were threatening tourist areas on the Algarve coast.

Belgium, Britain, Switzerland and the United States "have offered other types of assistance", notably financial aid, the NATO source said.

Portugal has also appealed to the European Union for disaster relief.

The wildfires which have swept through the country since the end of July have killed 15 people and destroyed 215,000 hectares (530,000 acres) of woodland -- an area the size of Luxembourg.

WAR.WIRE