WINNIPEG, Manitoba (AP) _ U.S. National Guard pilots went on standby Tuesday to help fight hundreds of fires that burned millions of acres of forests and brushwoods in Manitoba, Saskatchewan and Ontario.

Clouds of dense smoke hampered waterbombers and rescue efforts.

Thundershowers rumbled across the fire belt Tuesday, sprinkling light rain on the burning hinterland but failing to help control fires that have forced the evacuation of 20,500 people and blackened about 3.7 million acres. Many fires raged out of control.

Firefighters succeeded in holding fire lines around threatened communities, and officials said only a few houses and lodges have burned. There have been no reports of injuries or deaths.

Manitoba Natural Resources Minister Harry Enns said National Guard pilots stationed in North Dakota were put on standby and could arrive in Manitoba at any time they are needed.

''When the situation becomes such that we can't send up an aircraft because we haven't got a pilot, then we have these crews on standby,'' he said.

They would relieve waterbomber crews and pilots frustrated by smoke so thick it turned the sun into a reddish smudge.

Officials said Tuesday night that planes, trains and trucks had virtually completed the evacuation of people from fire-stricken areas of Manitoba.

But in Ontario 40 people were airlifted out of Kasabonika. Most of the evacuees were frome Manitoba but at least 350 people were removed from fire- threatened areas in Ontario and 900 were evacuated in Saskatchewan.

About 340 people from the small community of God's River in Manitoba arrived in Winnipeg Tuesday. They were placed in hotels and given vouchers to shop for clothes. Many had fled so hurriedly the only belongings they had were the clothes they were wearing.

Winnipeg relief workers organized activities and excursions for the refugees.

Officials said seven women from Minnesota on a 600-mile canoe trip did not realize the danger they faced until they paddled into Cross Lake, where surprised firefighters watched as they approached.

The women were flown to Thompson Tuesday and said they were anxious to continue their wilderness trip. The officials had no further information about the women.

The pall has made breathing difficult in the vast northern zone. Waterbombers could not see the fires blazing in brush and timberland.

''It's like dusk in the middle of the afternoon,'' Northern Affairs Minister Jim Downey said after returning from the sparsely populated region on Monday. Downey had flown with Manitoba Premier Gary Filmon to Thompson, The Pas and Flin Flon, where evacuees are being taken.

Filmon said he is considering sending medical staff from southern Manitoba to relieve doctors and nurses taxed by patients being treated for smoke inhalation at Thompson, a mining city of about 15,000. It has taken in more than 3,000 evacuees.

Hot, dry winds blew flames perilously close to settlements and mining towns.

''It's sometimes an hour-to-hour situation, waiting for the smoke to lift to a point where a plane can land,'' said provincial government spokeswoman Diana Soroka.

Thompson was many miles from the nearest fire Monday, but smoke shut down its airport temporarily.

The fire zone begins 250 miles north of Winnipeg and stretches 220 miles from northeastern Saskatchewan to northwestern Ontario.

Officials estimated 224 fires were burning in Manitoba and 165 in Saskatchewan on Monday, but said smoke obscured some blazes. The fires became a serious problem last week and evacuations began Thursday.

Soroka said some evacuees were being taken by plane, train and car to Brandon and Winnipeg in the south: ''We're taking them wherever the room is.'' Military transport planes, helicopters, hoses, pumps, tents, and other materials have been pouring in from other parts of Canada and the United States.

About 1,600 firefighters, 53 helicopters, 11 waterbombers, nine fixed-wing aircraft and several pieces of heavy equipment, such as bulldozers, have been thrown into the effort.

It's estimated $25 million will be spent by the end of the week, compared with $15 million for the whole season last year.