BUCHAREST, Romania — President Bush declared yesterday that he would not trade away his support for bringing Ukraine and Georgia into NATO in exchange for Russia dropping its opposition to an American missile defense network in Central Europe.

“There’s no trade-offs. Period,” Mr. Bush said, stating that it was a “misperception” that he was willing to make such a bargain.

The president said after meeting with the Ukrainian president, Victor Yushchenko, in Kiev, that he told President Putin of Russia in a recent telephone call Moscow had “nothing to fear” if the alliance eventually extends a welcome to the two countries, which once were part of the Soviet Union.

The question of whether to take the initial steps that could lead to Ukraine and Georgia joining the North Atlantic Treaty Organization has become a dominant issue in the days leading up to an alliance summit that begins today in Romania. Bush arrived late yesterday in Bucharest to attend the meeting.

The future of the missile defense system, which America wants to build in Poland and the Czech Republic as a shield against warheads launched from Iran or elsewhere in the Middle East, also is likely to be a central topic when Mr. Bush visits with Mr. Putin in Sochi, Russia, this weekend. The meeting is likely to be their final face-to-face conference before Mr. Putin leaves the presidency May 7.

Mr. Putin and other Russian officials have objected that the system, set close to Russia’s borders in former communist bloc nations, would threaten Russian rockets.

For Mr. Bush, the Bucharest summit provides an opportunity to draw attention to the changes that have taken place in the alliance since his presidency began in 2001, its reach now covering a wide swath of Eastern Europe from the Baltic to the Black Sea that during the Cold War was part of the Warsaw Pact.

Mr. Bush says in excerpts of a text distributed by the White House for a speech he plans to give today that extending the initial opening to Ukraine and Georgia would signal to their citizens “that if they continue on the path of democracy and reform, they will be welcomed into the institutions of Europe.” It would signal to the wider region, including Russia, that the two “are, and will remain, sovereign and independent states,” he says.

He says that “the need for missile defense in Europe is real and it is urgent.”

Mr. Bush spoke yesterday in Kiev at a news conference with Mr. Yushchenko, who has been outspoken in favor of joining NATO. Mr. Yushchenko acknowledged that he did not yet have the support of a majority of his citizens.

Referring to that opposition, Mr. Bush took note of a demonstration by Ukrainian communists displaying red flags bearing the hammer and sickle and banners linking Mr. Bush and NATO to a one-word profanity.

The demonstrators were encamped Monday and yesterday at Independence Square, the central site of the “Orange Revolution” of 2004 that brought Mr. Yushchenko to the presidency.