Three more properties have been destroyed in Tasmania’s bushfires as conditions worsened around the state on Wednesday, particularly in the south-west and central plateau.

Conservationists have also called on the prime minister, Scott Morrison, and Tasmania’s premier, Will Hodgman, to make an urgent request for international assistance to combat the “tragedy” unfolding in the state’s world heritage areas where valuable alpine vegetation is under threat.

“Fire is now at the very edge of the largest remaining forest of thousand-year old King Billy pines in the world, at Mount Bobs, and directly threatens Federation Peak and the entire Arthur Range,” the Wilderness Society said.

In a media conference on Wednesday, authorities said an approaching cool front would bring a small amount of rain but “nowhere near enough to be of any great assistance”.

By the afternoon, 14 emergency warnings were in place in relation to the Riveaux Road fire in the south-west and the Great Pine Tier fire in the central plateau.

The Tasmania fire service’s chief officer, Chris Arnol, said there was particular concern for communities in the Huon Valley area where fires were burning that had the potential to throw embers a kilometre ahead.

Crews were also working on a fire at Jimmy’s Hill near the township of Murdunna where the wind could push the blaze toward the Arthur highway.

“We know there are more challenging days ahead,” he said. “We’re asking people in fire affected areas to take a good decision about whether they stay and defend or whether they leave.

“We’re recommending that people now leave.”

Arnol said more than 520 crew were working on fires and 173,000 hectares had been burnt. He said the number of properties lost in the fires had now increased to six.

The state is facing more challenging conditions in the days ahead, despite a forecast drop in temperatures. It is expected the change will bring little rainfall and a change in wind direction.

The Bureau of Meteorlogy’s rainfall data for January shows most of Australia averaging 20% or less than its average rainfall for the month.

Amid the threats to properties, environment groups were warning of an unfolding crisis for Tasmania’s world heritage forests.

The Wilderness Society said the state’s firefighting crews were working to capacity protecting communities and properties and that international specialists should be brought in to assist with water-bombing of the state’s remote alpine areas.

“The scale of the fires, with over 2.5% of Tasmania burnt or burning, has overwhelmed firefighting capacity to the extent that little is available to combat remote fires in the Tasmania wilderness world heritage area,” the organisation said.

Vica Bayley, the Tasmania campaign manager for TWS, said “the impacted area in the world heritage area is significant already: many tens of thousands of hectares and growing every day and there’s no end in sight.

“We’d like to see the governments make the call now for additional international resources because it seems this emergency is going to drag on longer and threaten both properties and these irreplaceable wild places and we’ve got an obligation to protect both.”

Comment was sought from both the Tasmania and federal governments.

Earlier on Wednesday Tasmanians were being asked to conserve water, with TasWater saying mains supply was not guaranteed during times of fire.

“Fire conditions are expected to continue over this week and it is critical we have water available for the Tasmania fire service and residents defending their homes,” spokesman Mark McConnon said. “We would encourage everyone to make sure they use water efficiently.”

Police warned those who do leave to secure their homes after a 42-year-old man was charged with a number of burglary and stealing offences on Monday evening.