The U.S. military and intelligence agencies stand ready to help Russia with security for the Winter Olympics but have yet to be asked despite Moscow's demand Monday for a "collective" response to back-to-back suicide bombings in Volgograd.
"We're not aware of any request for assistance from the Russians," said Army Col. Steve Warren, a Pentagon spokesman.
In Hawaii, where President Obama was vacationing, National Security Council spokeswoman Caitlin Hayden said the U.S. was waiting for the call from Russian President Vladimir Putin.
"The U.S. government has offered our full support to the Russian government in security preparations for the Sochi Olympic Games, and we would welcome the opportunity for closer cooperation for the safety of the athletes, spectators, and other participants," Hayden said in a statement.
Terrorism "can only be stopped collectively," the Russian Foreign Ministry said in a statement following the suicide bombings Sunday and Monday that killed more than 30 people in the southern Russian city of Volgograd, about 450 miles northeast of the site of the Winter Olympics in Sochi.
The Ministry said the bombings of the main train station and a trolleybus in Volgograd were similar to recent terror attacks in the U.S., Syria, Iraq, Libya, Afghanistan, Nigeria, and other countries.
The U.S. mention appeared to be a reference to the Boston Marathon bombing last April that killed three and injured more than 260. The main suspects were two brothers, Dzokhar and Tamerlan Tsarnaev, from the Chechen Republic of Russia's troubled North Caucasus region.
No group has yet claimed responsibility for the Volgograd attacks, but a group called the Caucasus Emirate has threatened to use "maximum force" to disrupt the Olympic Games in Sochi in February.
In July, Doku Umarov, leader of the Caucasus Emirate, issued a statement saying that the "plan to hold the Olympics on the bones of our ancestors, on the bones of many, many dead Muslims buried on the territory of our land on the Black Sea, and we Mujahedeen are obliged not to permit that -- using any methods allowed us by the almighty Allah."
Volgograd could have been targeted because of its status as a major rail hub. Travelers by rail from Moscow and other areas must pass through Volgograd on their way to Sochi.
The Russian Foreign Ministry statement said that "A strike, cynically planned on the eve of New Year celebrations, is another attempt by the terrorists to open a domestic front, spread panic and chaos, cause interfaith strife and conflicts within the Russian society."
The terror attacks rattled the International Olympic Committee and several participating nations, whose previous main concern was Russia's increasingly harsh discrimination against gays and lesbians.
"I am certain that everything will be done to ensure the security of the athletes and all the participants of the Olympic Games," International Olympic Committee President Thomas Bach said in a statement.
However, Australian Foreign Minister Julie Bishop warned that her nation's athletes might be banned from participating in Sochi if the security situation deteriorated.
"These terror attacks are shocking and a reminder of the ongoing battle to confront extremism in all its forms worldwide," Bishop said. "We don't want to lightly prevent our athletes participating in any event for which they have trained for years, but their safety and the safety of their families and other spectators is of the utmost concern."