ANCHORAGE, Alaska (Reuters) - Smoke from wildfires in Alaska could cause very unhealthy air quality conditions and low visibility over the weekend in Anchorage, the state’s largest city, officials said.

Smoke from the 56,200-acre (22,700-hectare) Swan Lake Fire on the nearby Kenai Peninsula has slowed traffic to a crawl on some streets and obscured views of the mountains from the city, which is home to some 300,000 people.

The blaze, which was sparked by a June 5 lightning strike in the Kenai National Wildlife Refuge, was one of several that have burned close to cities in Alaska this year.

In Anchorage and in the Matanuska-Susitna Borough, a populated area just north of the city, air quality could fall to “very unhealthy” levels over the weekend, state environmental officials cautioned on Friday.

Early on Saturday, measurements of so-called PM 2.5, tiny particles that include dust, dirt, soot and smoke, were at moderate levels in Anchorage, according to the website for the state’s Division of Air Quality.

The smoke has forced officials to limit traffic on one major highway to a single lane led by a pilot car, causing major jams in one of Alaska’s most popular recreation and tourism areas.

“In the Kenai Peninsula, there’s one road in and one road out,” said Tim Mowry, a spokesman for the Alaska Division of Forestry. “If the fire impacts that road, it’s a big impact.”

Local visibility was expected to fall to a quarter of a mile or less over the weekend, the National Weather Service said.

Communities further north are also facing fire threats.

Residents of one subdivision in the Fairbanks area, about 300 miles (480 km) north of Anchorage, were put on evacuation alert late on Friday and told to be ready to flee the Shovel Creek fire, which is estimated to cover 2,300 acres (930 hectares).