* Problems not at critical level -local officials quoted
* U.S. Air Force delivers coolant to nuclear plant
* Thousands of residents evacuated as precautionary measure (Adds researchers' quotes, background)
By Osamu Tsukimori and Kiyoshi Takenaka
TOKYO, March 11 (Reuters) - Thousands of residents were evacuated from an area around a nuclear plant in Japan after a powerful earthquake raised fears of a radiation leak, but local officials said problems with the reactor's cooling system were not at a critical level.
There were no signs of a radiation leak, but the U.S. air force delivered coolant to the nuclear plant to avert a disaster within the wider disaster of the biggest earthquake on record to hit the country.
Experts said there could be leakage if water levels in the Fukushima reactor -- some 240 km (150 miles) north of Tokyo -- fell and the temperature of the nuclear rods rose, but that this danger did not appear to be imminent.
Work has begun on restoring the reactor's cooling function, Jiji news agency quoted the Trade Ministry as saying. Media quoted Fukushima prefecture as saying that water levels were 3.4 metres above the fuel rods at Tokyo Electric Power's Fukushima Daiichi No.2 reactor.
"Even if fuel rods are exposed, it does not mean they would start melting right away," said Tomoko Murakami, leader of the nuclear energy group at Japan's Institute of Energy Economics.
"Even if fuel rods melt and the pressure inside the reactor builds up, radiation would not leak as long as the reactor container functions well."
Residents that live within a 3 km radius of the plant have been told to evacuate from the area, Chief Cabinet Secretary Yukio Edano told a news conference. "The government is making every effort to restore the cooling system."
Kyodo news agency said 3,000 residents were being evacuated.
But Mark Hibbs, a nuclear expert at the Carnegie Endowment for International Peace, warned the situation could turn grave. "This is no laughing matter," he said.
TEPCO confirmed that water levels inside the reactors at its Fukushima-Daiichi nuclear plant were falling but it was working to maintain water levels to avert the exposure of nuclear fuel rods.
The company has been trying to restore power to its emergency power system so that it could add water inside the reactors, a TEPCO spokesman said.
"There is a falling trend (in water levels) but we have not confirmed an exposure of nuclear fuel rods," a TEPCO spokesman said.
Reactors shut down due to the earthquake account for 18 percent of Japan's nuclear power generating capacity.
Japan's nuclear power sector produces about 30 percent of the country's electricity and has been rocked periodically over the past decade by safety concerns. Many reactors are located in earthquake-prone zones such as Fukushima and Fukui on the coast.
Japan has also told the U.N. nuclear watchdog, the International Atomic Energy Agency, that a heightened state of alert was declared at the Fukushima facility.
TEPCO had been operating three out of six reactors at the Fukushima Daiichi nuclear plant at the time of the quake, all of which shut down.
The spokesman added that there were no concerns of a water leak for the remaining three reactors at the plant, which had been shut for planned maintenance. (Additional reporting by Risa Maeda in Tokyo and Fredrik Dahl in Vienna; Writing by Edwina Gibbs; Editing by Edmund Klamann and John Chalmers)