US regulators on Friday ordered operators of nuclear reactors to enhance safety measures in the event of a major accident, two days ahead of the anniversary of Japan's Fukushima crisis.

The Nuclear Regulatory Commission ordered all US plants to provide enough safety equipment for each reactor, ensuring that operators can respond simultaneously to multiple emergencies at every site.

The operator also required better protection for security equipment and, in the case of boiling water reactors such as those at Fukushima, upgrades in venting systems to prevent core damage during a serious accident.

"The commission has taken a significant step forward on our post-Fukushima efforts," its chairman Gregory Jaczko said in a statement.

The regulators gave companies until the end of 2016 to carry out the upgrades. Many reactor operators say they have already been improving safety since the Fukushima disaster.

Japan was rattled on March 11, 2011 by one of the strongest earthquakes in modern times which sent a tsunami crashing into the Fukushima Daiichi plant, forcing tens of thousands of people to flee as radioactivity spewed into the air.

The nuclear crisis did not directly claim any lives, although more than 19,000 people were killed by the force of the tsunami in Japan's worst post-World War II disaster.

Among its orders, the Nuclear Regulatory Commission also told operators to install enhanced equipment to monitor water levels in spent fuel pools, which nuclear plants need to keep cool.

Jaczko, testifying before Congress days after the tsunami, said that the Fukushima Number Four reactor's pool for spent fuel had dried up, meaning that radiation would likely spike. The assessment turned out to be inaccurate.

The commission last month approved the first new nuclear power reactors in the United States in decades, despite objections by Jaczko that safety lessons from the Fukushima crisis had not been fully addressed.

The safety orders will apply to the two newly approved 1,100 megawatt reactors, which Westinghouse-Toshiba plans to build in Georgia.