(Updates with Rice, more Pentagon, analyst comments)

By Tabassum Zakaria

WASHINGTON, Aug 13 (Reuters) - President George W. Bush on Wednesday ordered the Pentagon to deliver aid and sent Secretary of State Condoleezza Rice to visit Georgia in the strongest U.S. show of support for its ally since a conflict with Russia erupted.

Bush, who spoke with Georgian President Mikheil Saakashvili earlier in the day, demanded Moscow end the crisis, abide by an agreed ceasefire and withdraw Russian troops sent to Georgia after fighting began last week.

"The United States of America stands with the democratically elected government of Georgia. We insist that the sovereignty and territorial integrity of Georgia be respected," Bush told reporters at the White House.

"Russia must keep its word and act to end this crisis."

Georgia last week sent forces to retake South Ossetia, a pro-Russian province that threw off Georgian rule in the 1990s. Moscow responded with a massive counter-attack that overwhelmed Georgian forces.

Russia also moved beyond separatist areas and into Georgia proper, prompting Rice to say Moscow "seriously overreached."

Russia and Georgia agreed to a ceasefire but reports persisted that Moscow's military activities continued.

Flanked by Rice and Defense Secretary Robert Gates in the Rose Garden, Bush said he was concerned about reports that Russian forces were blocking roads and access to the port in Poti, and blowing up Georgian vessels.

While Moscow has denied violating the cease-fire, Bush cited reports that Russian units had taken up positions inside Georgia that allow them to threaten Tbilisi.

MILITARY FLIGHTS

A U.S. C-17 military aircraft carrying supplies arrived in Georgia and a second flight was planned for Thursday.

Bush said the United States expected Russia to allow humanitarian supplies into Georgia and ensure that all lines of communication and transport remain open.

Analysts said the United States was trying to use the military flights to demonstrate strength to both Georgia, which has sought a stronger response from Washington, and Russia while minimizing the risk of getting drawn into the conflict.

When asked about the risks of having U.S. and Russian militaries operating in such proximity, Pentagon spokesman Geoff Morrell said the United States had been careful to let Russia know about every flight it planned.

"We do not want there to be any potential misunderstanding or miscalculation," he said.

At the State Department, Rice said Russia faced deeper isolation if it violated a cease-fire with Georgia.

"I really do believe that the Russians understand that pushing the envelope here would have significant consequences for Russia's standing in the international system," she said.

Rice, who leaves on Wednesday, travels first to France to meet President Nicolas Sarkozy and then to Tbilisi. The United States backed French-led efforts to broker an end to the conflict, Bush said.

Charles Kupchan, a senior fellow for Europe studies at the Council on Foreign Relations, said the relative absence of U.S. diplomacy until now in the crisis had been "striking".

"That may have been the product of the fact that the Europeans have more leverage with the Russians on Georgia than does the United States," he said.

Rice bristled when asked about comments by Russian Foreign Minister Sergei Lavrov that the Georgian leadership was a "virtual project" of the United States. Moscow has been angered by strong U.S. backing for Georgia's bid to join NATO.

"Georgia is a democratic government in the Caucasus that has elected its leaders. To call it a project of anyplace, of anybody, perhaps belies more about what Russia thinks about its neighbors than what it thinks about U.S. policy," Rice said. (Additional reporting by Susan Cornwell and Andrew Gray; Editing by Cynthia Osterman)

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