Russia has received numerous letters of condolence over Friday's derailment of a luxury express train, which left dozens of people dead.

Three carriages of the high-speed Nevsky Express train derailed late on Friday leaving at least 26 people killed and 96 injured. A total of 18 people are still listed as missing. The derailment was caused by an explosive device equivalent to 7 kg of TNT.

A second, weaker bomb had exploded on Saturday at 2:00 p.m. (11:00 GMT) at the site of the attack, but that it had not caused any injuries.

The U.S. said it was "deeply saddened by the terrible loss of life and injuries resulting from the reported derailment of a train between Moscow and Saint Petersburg."

"As authorities investigate the circumstances of this tragedy, our hearts go out to the families of those who lost loved ones, and we wish a speedy recovery to the injured," the White House said in a statement.

Leaders of ex-Soviet states, including Ukraine, Belarus, Azerbaijan, Estonia, Moldova, Lithuania, also offered their condolences to Russian leaders.

"On behalf of the Ukrainian people... I ask you to deliver sincere condolences to the families of those killed and words of support to all injured," Ukrainian president's press service quoted Viktor Yuschenko as saying.

Prime Minister Yulia Tymoshenko and Parliamentary speaker Volodymyr Litvin have also sent letters to their Russian counterparts.

Belarusian President Alexander Lukashenko said in a letter to the Russian president that the accident "echoed with pain in the hearts of Belarusians, who sincerely share your grief."

The Spanish government has also offered its condolences and said it "condemned the terrorist attack which led to the train derailment."

"The government also expresses to the friendly peoples of Russia its solidarity and full support in the fight against terrorism," the Spanish foreign ministry's press service said in a statement.

MOSCOW, November 28 (RIA Novosti)