WEST ALLIS, Wis.  —  President Bush (search) on Friday called the siege of a Russian school "another grim reminder" of terrorists tactics and asked Americans to pray for the victims.

Hundreds of children and adults were injured and well over 150 people may have died, Russian authorities said Friday as commandos battled separatist rebels at the school in southern Russia (search).

Bush, who rarely departs from his scripted stump speech to address current events, condemned the hostage-taking during a re-election rally.

"This is yet another grim reminder of the length to which terrorists will go to threaten this civilized world," Bush told thousands of supporters. "We mourn the innocent lives that have been lost, we stand with the people of Russia, we send them our prayers for this terrible situation."

High-ranking advisers traveling with Bush said they were having difficulty gauging the situation in Beslan, and were relying largely on news reports.

White House spokesman Scott McClellan said the responsibility for dozens of lost lives rests with the terrorists.

"There is no justification for the taking of innocent life, and the barbaric nature of this terrorist act is despicable," McClellan said.

He added, "The responsibility for the tragic loss of life rests with the terrorists. The United States stands side-by-side with Russia in our global fight against terrorism."

Bush was briefed on developments in Russia Friday morning before his first re-election rally of the day, in Pennsylvania.

Bush spoke with Russian President Vladimir Putin (search) on Wednesday, after the separatist rebels took over the school, and U.S. officials have continued since then to express "our sympathy and our solidarity" with the Russians, said State Department spokesman Richard Boucher.

"We grieve with Russia on this tragic day," Boucher said.