WASHINGTON—President Bush put the U.S. more firmly than ever on Georgia’s side in its conflict with Russia on Wednesday, sending humanitarian aid on American military planes to help the embattled ex-Soviet republic and displaying growing impatience with Moscow’s aggression.

Six days into the fighting in the tiny, impoverished country wedged between Russia and Turkey on the Black Sea, Bush said Moscow’s apparent violation of a cease-fire agreement puts its aspirations for global acceptance at risk. In brief but stern remarks from the White House, the president demanded that Russia end all military activity inside its neighbor and withdraw all troops sent in recent days into Georgian territory.

Amid some fear that Russian troops may be setting up for some type of medium-term occupation of parts of Georgia or even have intentions to press on to its capital of Tbilisi, Bush promised to “rally the free world in the defense of a free Georgia.”

Bush postponed Thursday’s planned start of a two-week Texas vacation for a couple of days to monitor developments. He also dispatched Secretary of State Condoleezza Rice to Paris for talks with Europeans and then to the Georgian capital of Tbilisi “to demonstrate our solidarity with the Georgian people,” and announced that a massive U.S. aid effort for devastated Georgians was already under way.

“This is not 1968 and the invasion of Czechoslovakia where Russia can threaten a neighbor, occupy a capital, overthrow a government and get away with it,” Rice said just hours before leaving for France. “Things have changed.”

Russian tanks on Wednesday rumbled into the Georgian city of Gori, a hub along the country’s main east-west highway, and Georgian officials said it was looted and bombed. An Associated Press reporter later saw dozens of tanks and military vehicles roaring south, deeper into Georgia. Bush also cited evidence that Russian forces have entered and taken positions in the port city of Poti and that Russia was blowing up Georgian vessels.

Rice said the developments, taken together, are “well beyond anything that is needed to protect Russian peacekeepers” who have been stationed in the separatist provinces of South Ossetia and Abkhazia since the early 1990s.

Later in the day, Georgian officials said the Russians pulled out of the western town of Zugdidi, near Abkhazia.

But a U.S. intelligence official said it is believed that Russians are consolidating their positions in South Ossetia and Abkhazia, which are loyal to Moscow.

The crisis erupted last week when Georgia tried to secure control over South Ossetia. Russia’s fierce military response expanded to Abkhazia along Georgia’s coast, and ended up on purely Georgian soil.

Bush’s statement Wednesday represented his clearest—though still unspecified—threat to Moscow.