WASHINGTON (Reuters) - Isolating Moscow over its incursion into Georgia does not seem to be a viable option for the Bush administration — Russia is just too important to the United States.

"It seems to me that the United States has to have a darned good reason to break off relations with Russia and go back to the dark days of the Cold War," said Charles Kupchan of the Council on Foreign Relations.

"I don't believe events thus far provide that reason ... Both sides need the other too much," he told Reuters.

If relations deteriorate to a Cold War level, a lot is at stake, from cooperation at the United Nations on curbing Iran and North Korea's nuclear ambitions to US access to Asia and Afghanistan. Russia is also an important energy supplier.

So far, Washington's response has amounted largely to rhetoric and it has taken only tiny steps to isolate Moscow, which sent troops into Georgia last week after Tbilisi tried to retake a separatist pro-Russian region that rejected Georgian rule in the 1990s.

The United States has excluded Moscow from discussions among the Group of Eight industrial nations over the Georgian crisis and canceled a naval exercise with Russia.

But Secretary of State Condoleezza Rice and President George W. Bush have made clear that Moscow's membership in global bodies such as the World Trade Organization could be in jeopardy if the military action continues.

Rice, a Soviet expert, did not go to Moscow this week when she went to France and Georgia, visiting Tbilisi on Friday to publicly demonstrate firm U.S. backing for a country Bush calls a beacon of democracy, in line with the administration's so-called freedom agenda.

The State Department insists Rice has been in contact often by phone with Russian Foreign Minister Sergei Lavrov since the crisis began, but she has spent much more time reassuring Georgian President Mikheil Saakashvili, who called Russians "evil" and "barbarians" at a joint news conference with Rice.

Bush also took aim at the Russians on Friday, accusing Moscow of "bullying" and damaging its international standing by sending its military into Georgia.

But experts said the United States could not push too hard because punishing and isolating Russia was not the solution.

"We have never profited much by not talking to people with whom we don't have common ground," said James Collins, a former US ambassador to Russia.

"I have never believed we are very effective at trying to play the game of reward and punishment with a nation as big as Russia," added Collins, now with the Carnegie Endowment for International Peace.

Relations have deteriorated steadily in recent years between the two countries, especially over US plans for a missile defence shield in Poland and the Czech Republic, US backing for Kosovo's independence from Serbia and Georgia's proposed NATO membership.

The signing of a long-stalled US agreement with Poland on the missile defense shield, which Russia sees as a threat, will further complicate US-Russia ties.