Mr Bush says the US stands by Georgia's 'democratically elected government'
President George W Bush has said the US will use military aircraft and naval forces to deliver aid to Georgia following its conflict with Russia.
Speaking in Washington, he expressed concern about reports of continuing Russian action in Georgia, and urged Russia to respect a ceasefire accord.
Mr Bush hinted that Russia could be jeopardising its international ties.
The first US C-17 military aircraft has landed in Tbilisi carrying medical and other supplies.
The Kremlin said the US must choose between partnership with Moscow, or with the Georgian leadership.
"At some time it will be necessary to choose between supporting this virtual project and [a] real partnership on questions which actually require collective action," said Russian Foreign Minister Sergei Lavrov.
The BBC's Caroline Wyatt, in Moscow, says the Kremlin's reaction suggests they have been bitterly stung by Mr Bush's comments.
But she says Mr Lavrov spoke in the knowledge that the US is unlikely to back up its verbal support for Georgia with any more concrete action.
A French-brokered ceasefire has been in place between Russia and Georgia since Tuesday, but each side has accused the other of breaking the accord.
Mr Bush said Russia's actions had "raised serious questions about its intentions in Georgia and the region".
"To begin to repair the damage to its relations with the United States, Europe and other nations, and to begin restoring its place in the world, Russia must keep its word and act to end this crisis."
He said he had ordered a series of steps to demonstrate "solidarity with the Georgian people", including sending Secretary of State Condoleezza Rice to Tbilisi later this week, and launching a "vigorous and ongoing" humanitarian mission.
A C-17 aircraft with humanitarian supplies was already on its way to Georgia, Mr Bush said, and in the following days military aircraft and naval forces would deliver humanitarian and medical supplies.
The BBC's Natalia Antelava, in Tbilisi, says Mr Bush's speech was the first piece of good news the Georgian government had received for days.
But she said Tbilisi's schools and nurseries were crammed with refugees, many of whom were angry with their leaders for dragging them into a conflict with Russia.
Following Mr Bush's statement, Ms Rice, who will hold talks in France before heading to Tbilisi, also had tough words, saying Russia had "seriously overreached" itself.
Russian and Georgian forces have been seen on the main Gori-Tbilisi road
She told a news conference: "This is not 1968 and the invasion of Czechoslovakia where Russia can threaten a neighbour, occupy a capital, overthrow a government and get away with it. Things have changed."
Mr Lavrov responded robustly to US criticism of Russia's continuing military action, admitting that his troops were still inside Georgia near both Gori and the town of Senaki, near the other secessionist region of Abkhazia.
He said Russian peacekeepers were dismantling a "huge amount of armaments, ammunition and explosives" that had been left unattended.
"This arsenal has to be defused of course so that it cannot pose any threat to civilians," he said.
'Massacres on our doorstep'
France, which currently holds the EU presidency, has been spearheading diplomatic efforts to solve the crisis.
President Nicolas Sarkozy visited both Tbilisi and Moscow on Tuesday, and succeeded in getting both sides to agree to the principles of a peace plan.
Russian President Dmitry Medvedev had announced a halt to military action shortly before meeting Mr Sarkozy.
EU foreign ministers have been discussing the peace plan, and have agreed to send a group of monitors or peacekeepers to Georgia to monitor the ceasefire - but they want the UN to back the proposal first.
"The European Union cannot be indifferent to this war, these massacres on our doorstep," said French Foreign Minister Bernard Kouchner.
UN Secretary General Ban Ki-moon said the UN was ready to facilitate international talks and contribute to possible peacekeeping arrangements in the region.
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