Mr Bush met his Romanian counterpart before the start of the Nato summit
US President George W Bush has repeated his call for Nato to expand eastwards.
Speaking in Romania ahead of Nato's summit in Bucharest, he said the former Soviet states of Ukraine and Georgia should be offered paths to membership.
Mr Bush also said he "felt good" about Nato allies' reaction to appeals for more troops to be sent to Afghanistan.
Russian foreign minister Sergei Lavrov said his country would not sulk over the expansion plans - but stressed that they would not "go unanswered".
"This will not be left without an answer, I can assure you," Mr Lavrov told parliament in Moscow.
"But we will respond pragmatically, not like small boys in school who sulk at those who bully them, run out of the room, slam the door and start crying in the corner.
"We must concentrate on increasing our economic power and taking our defence capabilities to a higher technologic level."
Mr Bush stressed that Georgia and Ukraine should be offered "a clear path" towards the goal of Nato membership.
"We must make clear that Nato welcomes the aspirations of Georgia and Ukraine for membership in Nato," he said.
He added that "Nato membership must remain open to all of Europe's democracies that seek it, and are ready to share in the responsibilities of Nato".
The prospect of more ex-Soviet states joining Nato is opposed by Russia, while Nato members France and Germany have warned that it will worsen relations with Moscow.
Arriving for the summit, Germany's Chancellor Angela Merkel said Ukraine and Georgia should have a long-term prospect of Nato membership, but that now was not the time for Membership Action Plans (MAP) to be extended to them.
"We have reached the conclusion that it is too early to give both countries MAP status," she said, but added: "The message is: the door is open. We see a perspective for accession. We want to help both countries go toward MAP."
French prime minister Francois Fillon said on Tuesday of Ukrainian and Georgian membership: "We think that it is not the correct response to the balance of power in Europe."
Mr Bush spoke at the Black Sea resort of Neptun on Wednesday, where he met Romania's President Traian Basescu.
In a joint press conference, Mr Bush said he "strongly believed" Croatia, Albania and Macedonia should be able to join Nato as full members.
The huge Parliament Palace in Bucharest is hosting the summit
He added that Membership Action Plans should be extended to Ukraine and Georgia, and strongly supported calls by Montenegro and Bosnia-Hercegovina for "intensified dialogue" on membership. "We ought to open the door to closer co-operation with Serbia," he also said.
However, Greece has said that, if no last-minute compromise is reached, it would veto an invitation to Macedonia to join Nato because it objects to the country's name.
Macedonia is also the name of a province in northern Greece, and Athens worries that this could imply a claim on its territory.
Earlier, Nato Secretary General Jaap de Hoop Scheffer told the BBC he supported membership for the former Soviet states.
"There's no way that the door will be locked for Ukraine and Georgia," he said.
"The Nato Treaty very clearly states that European democracies fulfilling their criteria for Nato membership are welcome."
Mr de Hoop Scheffer said he understood Russian concerns but added that the "final decision will be taken by the allies and not by anybody else".
The three-day summit of leaders from the 26-nation alliance in the Romanian capital, Bucharest, is being billed as the biggest in the alliance's 59-year history.
Nato enlargement and efforts to rally support for the Nato-led force in Afghanistan are the topics expected to dominate the agenda.
Mr Bush also spoke of his forthcoming talks with outgoing Russian president Vladimir Putin - whom he will meet at the summit, and again at private talks in the Russian Black Sea resort of Sochi at the weekend.
The US leader said he would make clear to Mr Putin that "the Cold War is over and Russia is not our enemy".
"This is a good chance for me to sit down and have yet another heart-to-heart with him," he said.