BERLIN — Russian oil supplies to the Czech Republic have been cut by almost half after Prague agreed to host part of America’s controversial missile defense shield.

Czech officials have sought an explanation from Moscow about the reduction in supply, fearing that it could be retaliation for the radar base deal, signed last Tuesday.

A spokesman for the Czech trade ministry, Tomas Bartovsky, said yesterday that Russia had ruled out “political reasons” for the reduction and had blamed negotiations between suppliers for the problem.

But the Czech prime minister, Mirek Topolanek, was sceptical.

“I want to believe reasons which the Russian supplier states are only technical,” he said.

The drop in Russian oil supplies coincided with the visit of Secretary of State Rice to Prague to sign the missile shield agreement, which Moscow fiercely opposes.

As the ink dried on the deal, which will provide radar control for an American silo of interceptor missiles due to be based in Poland, oil flow through the Druzhba — or Friendship — pipeline began to ebb.

Business leaders in the Czech Republic, which gets 70% of its oil from Russian deliveries, were reported to be unsettled after the cuts, fearing a prolonged energy war with Russia. Russia’s vast natural resources have played a key part of its foreign policy strategy in the recent past.

Both Ukraine and Georgia, which have recently sought to distance themselves from Russian influence, have accused Moscow of “energy blackmail” designed to curb their independent spirit.

Russia briefly cut off all gas supplies to Ukraine in 2006, following a price dispute.

At the same time, President Saakashvili of Georgia accused Russia of being behind explosions, which cut energy supplies. Both nations are candidates for NATO membership, which Russia opposes.

But Mr. Topolanek said that the energy cuts posed “no threat for the citizens” because of his country’s reserves, and its ability to source more oil from a western European pipeline.

America has long insisted that the missile shield, though based in Eastern Europe, is directed at “rogue states” such as Iran, to intercept long-range, potentially nuclear, missiles fired at America. But Russia has consistently viewed the project as a threat to its nuclear arsenal and says it would upset the balance of power on the continent.

Poland, which also receives oil through the Druzhba pipeline, but has not yet signed a deal with America to host the missile base, has reported no drop in supplies.

It remains cautious about signing the deal, demanding that America upgrade Polish armed forces with Patriot missiles as part of the agreement. Lithuania has been cited as a potential replacement for Poland, should Warsaw pull out of the deal. It would be immune to energy retaliation for any deal because Russia cut off its supplies through the Druzhba pipeline in 2006.