OTTAWA (AFP) - Chemical and heavy metal contamination will delay the return of 9,000 residents from Canada's oil sands region who fled a huge wildfire about a month ago, senior officials announced on Monday.

Alberta Premier Rachel Notley told a press conference that a voluntary phased re-entry would begin on Wednesday as planned "for the vast majority of residents of Fort McMurray and the surrounding communities". But residents of three hard-hit neighborhoods - Abasand, Beacon Hill and Waterways - will have to wait.

"They should be making plans to stay elsewhere at least in the short-term...until further remediation and clean-up of the neighborhoods is complete," Notley said.

Most of the homes in these neighborhoods were burned to the ground when fires swept through at the beginning of May.

However, the more than 500 houses and apartments not damaged in the wildfire have been deemed unsafe for habitation until they can be cleaned up, said Alberta province's chief medical health officer Karen Grimsrud.

Tests of air, soil and ash in these neighborhoods revealed the presence of chemicals "that are a risk to residents exposed to them," she said.

The chemicals can cause skin and respiratory irritation and burns. Heavy metals such as arsenic have contaminated the ground.

A 30-kilometre temporary fence was built around the three neighborhoods to keep people out.

The wildfires had forced the evacuation of about 100,000 residents of Fort McMurray and the partial shutdown of oil sands operations to the north.

While continuing to grow - to 580,000 hectares at last estimate - the blaze has moved away from populated areas.

Air pollution due to smoke has also mostly abated.

Still, Notley said she would ask her legislature to extend a state of emergency to the end of June in order complete the cleanup and process an orderly return of residents.