MOSCOW, Sept 11 (AFP) - Russia put troops on alert Tuesday after a spate of unprecedented attacks rocked Washington and New York as President Vladimir Putin told his US counterpart he supported a tough response to the strikes.
"The series of barbaric acts directed against innocent people fills us with indignation and revolt," said Putin in a telegram to US President George W. Bush.
"Such inhuman acts must not go unpunished," he said, according to the Interfax news agency.
Putin later telephoned US national security advisor Condoleezza Rice to ask her to convey his condolences to Bush.
Two planes crashed into the World Trade Center in New York, destroying it entirely, and another ploughed into the Pentagon in Washington, in the worst wave of terrorism to hit the United States.
Putin convened an emergency meeting with his "power ministries," including foreign, interior and defence ministers and head of the domestic intelligence agency to give them special instructions, the RIA-Novosti news agency said.
The country's air force meanwhile announced a series of emergency anti-terrorist measures to protect Russian air space.
"The Russian anti-aircraft defenses are constantly on a state of war readiness. In connection with the events in the United States, a series of anti-terrorist measures are being put into action," the air force said, as quoted by Interfax.
Interior ministry forces were placed on a state of alert and security reinforced around key sites across the country while Russia halted all plane departures to the United States.
Putin later said in a televised address that the world should unite to combat the threat of terrorism, "the plague of the 21st century,"
These strikes are "an unprecedented aggression by international terrorism" which "go beyond the borders of the United States," the Russian president said.
"This is a challenge to the whole of humanity," he added, saying the unprecedented terrorist acts underlined "the relevance of the Russian proposal to unite efforts to fight terrorism."
Russia is waging a military crackdown in the separatist republic of Chechnya, which it describes as an "anti-terrorist" operation.
Former Soviet president Mikhail Gorbachev said the attacks were a catastrophe for mankind.
They represent "not only a national tragedy (for the United States), but a tragedy for the whole of mankind," he said in a message of condolence to the US administration, as quoted by Interfax.
"In my opinion, countries and nations must unite against this folly, and do everything to stop the escalation of terror," Gorbachev added.
The US embassy in Moscow meanwhile sent Americans in Russia Tuesday an e-mail advising them to exercise caution.
And at least three money exchanges offices in the Russian capital refused to accept dollars. Cashiers said they had been given instructions not to accept the US currency, universally used as a safe haven by Russians.