The two companies, long scorned by green groups, have been working for years to put out a more environmentally friendly message.
Their efforts have been driven by growing sensitivity to green issues among customers in markets such as Europe, Japan and the US.
But in a report, WWF researchers say deforestation in the Sumatran province of Riau, where the groups have pulp mills, has accelerated in recent years, in spite of conservation pledges from APP and APRIL.
The report raises questions over whether either company can meet their promises to rely wholly on plantation wood for their Sumatran mills by the end of this decade.
According to WWF, more than 200,000 hectares of natural forest - an area 35 times the size of Manhattan - were lost in the province of Riau last year, up from 73,000 hectares in 2002, at a time when APP and April were facing financial difficulties.
Pulp makers have again begun pulping forest at a rate equal to the peak of the boom that preceded APP's 2001 default on $14bn in debt. According to WWF, APP pulped about 80,000 hectares of natural forest last year while April pulped 90,000.
APP and April have pledged to rely on acacia plantations to supply fibre for their mills in Sumatra. But WWF said the "best available estimates for 2004 and 2005" showed that the companies still relied on natural forest for 70 per cent of their wood supply.
APP said it could not immediately comment on the report. A.J. Devanesan, April's president and chief operating officer, said the company had been increasing the amount of plantation wood it used steadily.