KIEV, Aug. 4 – Ukrainian officials were told to suspend vacations and to arrive for an emergency briefing focusing on preventing the spread of wildfires that have been wrecking havoc on neighboring Russia.

President Viktor Yanukovych ordered regional governors and government officials to cancel vacations and to arrive for the briefing, Unian news agency reported Wednesday citing a source.

Crimean Prime Minister Vasyl Dzharty on Wednesday asked the Defense Ministry to postpone any military exercises at shooting ranges in the peninsula amid fears they may cause the fire.

At least one major wildfire was reported in the Dnipropetrovsk region over the past two days, but the extremely hot weather has been creating favorable conditions for the fires to spread.

“We cannot allow what has happened in Russia,” Dzharty said. “President Yanukovych ordered to get the situation under control. We have to prevent the wildfires.”

Russia was especially badly hurt by the spread of the uncontrolled wildfires, underscoring a poor readiness of the government to react to emergency situations.

Moscow was engulfed Wednesday by the thickest blanket of smog yet this summer, a choking haze from wildfires that have wiped out Russian forests, villages and a military base.

Passengers on Moscow's subway said the eye-stinging haze hovered above the platforms, and City Hall warned of health risks from the smoke, which is carrying harmful gases including carbon monoxide.

To the east, firefighters focused on beating flames back from a top-secret nuclear research facility, according to The Associated Press.

Russian President Dmitry Medvedev fired several high-ranking military officials over what he called criminal negligence in fires that ravaged a military base.

Russia is suffering through its worst heat wave on record, a condition that has sparked forest and peat fires across its central and western regions that have killed at least 48 people in the past week.

Over the last 24 hours, firefighters have extinguished 293 fires, but 403 others have been spotted and more than 500 others have continued to rage over large swathes of countryside, some of them out of control, the Emergencies Ministry said.

Dry winds have sent clouds of smog over Moscow before, but Wednesday's was the worst yet, with the haze obscuring the capital's landmarks and penetrating the subway system.

Moscow's 10 million residents were cautioned to protect themselves against the polluting smog, which came from wildfires in forests and peat bogs to the south and east.

Some 400 kilometers to the east, about 2,000 army troops and emergency personnel were fighting back flames that surrounded Russia's top nuclear research facility in Sarov.

The situation there was "tense but not critical," Deputy Defense Minister Dmitry Bulgakov said after new robotic firefighting equipment was sent to the scene overnight.

"There is no threat to the Federal Nuclear Center, and there is no reason for worry," Bulgakov was quoted by the ITAR-Tass news agency as saying. The country's nuclear chief, Sergei Kiriyenko, was quoted as saying that all explosive and radioactive material had been moved off site as a precaution.

The top-secret facility is Russia's main nuclear research center and the birthplace of Soviet nuclear weapons.

Another risk of radioactive contamination stems from the forest fires sweeping through areas polluted by the 1986 explosion at the Chernobyl nuclear plant, Russian environmentalists said. The fires have the potential to send radioactive dust into the air, Nikolai Shmatkov of the World Wildlife Fund's Russia office and Vladimir Chuprov of Russian Greenpeace told The Associated Press.

But nuclear energy scientists said the danger came not from radioactivity but from fine particles in the smoke.

"The concentration of radioactive elements will be so negligible that the smoke itself will be many more times more dangerous than the radioactivity in it," Ravil Bakin of the Institute for Safe Development of Nuclear Energy told the AP. "Fine dust that contains chemical pollutants is the real danger and is much more poisonous than radioactivity." (tl/ap/ez)