Last month's devastating earthquake in central China could not have been predicted, a top U.S. seismologist said in support of Chinese experts trying to refute claims they ignored warning signs.

The U.S. Geological Survey's top expert on seismic conditions in China, Walter Mooney, said few earthquakes register with foreshocks that could warn monitors. Also, there are no reliable ways to predict earthquakes in the short term, even in high-risk areas, he says.

"Leading scientists in China said there were no pre-monitoring signals," Mooney told reporters during a visit to Beijing.

Mooney said hard-packed earth beneath the foothills of the Tibetan plateau hemmed in the shockwaves, preventing them from dissipating and intensifying their impact in the quake area.

The 7.9-magnitude earthquake in Sichuan province was especially damaging because it spread quickly along a fault line, he said.

Chinese seismologists have been defending themselves against accusations they ignored warning signs of the impending quake, which killed nearly 70,000 and left five-million homeless.

There has been rising public anger about the deaths of thousands of children when some 7,000 classrooms collapsed. Some of the destroyed schools were in areas where no other buildings were badly affected, causing parents and some engineers who surveyed the rubble to question a lack of steel reinforcement bars in concrete and the use of substandard building materials.

For the one-month anniversary of the May 12 earthquake on Thursday, hundreds of grieving parents blocked a road into an earthquake-flattened town.

Police tried to quell the situation by detaining volunteers, cordoning off schools and barring reporters from destroyed classrooms in some towns.

While Beijing relaxed restrictions against both domestic and foreign reporters in the days following the quake, the government has recently begun clamping down as questions are raised about corruption and poor construction.

With files from the Associated Press