(Adds U.S. presidential candidates statements)

By Jeremy Pelofsky

CRAWFORD, Texas, Aug 26 (Reuters) - U.S. President George W. Bush on Tuesday condemned Russia's decision to recognize Georgian rebel regions as independent and accused Moscow of escalating tensions with an "irresponsible decision".

He was reacting to Russian President Dmitry Medvedev's announcement he had signed decrees recognizing the independence of the breakaway regions of South Ossetia and Abkhazia.

Bush said the decision was inconsistent with a French-brokered cease-fire agreement Georgia and Russia signed earlier this month, as well as U.N. Security Council resolutions stating the two areas are part of Georgia.

"We expect Russia to live up to its international commitments, reconsider this irresponsible decision and follow the approach set out in the six-point (cease-fire) agreement," he said in a written statement from his Texas ranch.

"Russia's action only exacerbates tensions and complicates diplomatic negotiations," Bush said. "In accordance with United Nations Security Council resolutions that remain in force, Abkhazia and South Ossetia are within the internationally recognized borders of Georgia, and they must remain so."

In February, Russia strenuously objected when the United States and many European nations recognized Kosovo's independence from Serbia.

In separate statements, U.S. presidential rivals Barack Obama and John McCain joined in condemning Russia's decision to recognize the two rebel regions and said Moscow should be further isolated.

The conflict erupted when Tbilisi tried to retake the pro-Moscow separatist region of South Ossetia, which threw off Georgian rule in the 1990s. Russia responded with a counter-attack that overwhelmed Georgian forces and occupied much of the country. It has since partially withdrawn.

HUMANITARIAN AID OR ARMS?

Medvedev, in a BBC interview, accused the United States of shipping arms to Georgia on naval vessels under the cover of humanitarian aid. The White House said that was "ridiculous".

"I can assure you that these are purely humanitarian aid shipments that are going into Georgia and nothing else," White House spokesman Tony Fratto said.

A source close to the U.S. embassy in Tbilisi said U.S. warships have scrapped a plan to deliver relief supplies to the Russian-controlled port of Poti on Wednesday. The Pentagon had no comment.

The White House has warned Moscow of consequences for its actions but has refused to say what it might do.

U.S. Secretary of State Condoleezza Rice, traveling in the Middle East, said Russia's decision to recognize the regions puts it in opposition to several U.N. Security Council resolutions to which it is a party.

"I want to be very clear," Rice said. "Since the United States is a permanent member of the (U.N) Security Council, this simply will be dead-on-arrival in the Security Council" if Russia tries to introduce changes.

The United States has partnered with Russia on some key foreign policy issues, such as trying to convince North Korea to give up its nuclear weapons programs, and Fratto said he did not expect those to suffer from the rising tensions.

"It is not in Russia's interest to have a nuclear armed nation on its border, on the Korean peninsula," Fratto told reporters in Texas. (Additional reporting by Arshad Mohammed in Ramallah, Sue Pleming, David Morgan and Tabassum Zakaria in Washington; Editing by David Alexander and Cynthia Osterman)

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