BRUSSELS -- President Barack Obama started his day in Warsaw struggling to convince his friends in Central and Eastern Europe that the United States is being tough enough with Russia. He ended his day in Brussels, still struggling, but this time to persuade America's core Western allies to stay tough with Russia.

The dizzying contrasts underscored the challenges that Mr. Obama faces navigating the complicated waters of European politics as he tries to forge a unified stance against Russian aggression in Ukraine. On the defensive at home for a prisoner swap, he finds himself pressed overseas by some allies unsatisfied with his reassurances of resolve and others unimpressed with his arguments for action.

He arrived Wednesday in Brussels to have dinner with the leaders of the Group of Seven powers, who, at his urging, had excluded Russian President Vladimir V. Putin as punishment for his annexation of Crimea. But Mr. Obama's counterparts from Britain, France and Germany all ended up scheduling one-on-one meetings with Mr. Putin later on. French President François Hollande even arranged to have dinner Thursday with Mr. Putin just after having a separate dinner with Mr. Obama.

Not only were they unwilling to snub the Russian leader entirely, as Mr. Obama had sought, but they were also reluctant to go along with other efforts to isolate the Kremlin. Most notably, the French government repeated that it would go ahead with the $1.6 billion sale of powerful warships to Moscow, along with plans to train 400 Russian sailors in France this month. And other European leaders were cautious about setting further red lines threatening additional sanctions against Russia.

Obama aides repeated their opposition to the French sale Wednesday but tried to play down the leaders' disparate approaches. "The question is not whether they're meeting," deputy national security adviser Ben Rhodes said. "The question is what people are saying in those meetings. And our belief is that there needs to be a unified message."

The leaders used their dinner Wednesday to discuss what might set off another, more expansive, round of sanctions.

Some Europeans want to keep new sanctions in their pocket, as they put it, to impose only if Russia escalates the situation, while others say Moscow should avoid new penalties only if it proactively works to stop pro-Russian separatists in Ukraine.

After a long discussion, the leaders left the question largely unresolved. In a joint statement, they again condemned Moscow's actions and called on Russia to stop the flow of arms and fighters across its borders. But they did not specify what might prompt them to broaden their sanctions to target entire sectors of the Russian economy. Instead, they threatened "to impose further costs on Russia should events so require," without elaboration.

Unlike some other Western European leaders, German Chancellor Angela Merkel sided with the tougher line in a speech to her parliament before flying to Brussels. Mr. Putin "has to make his influence felt" with pro-Russian separatists who have attacked and seized government offices in eastern Ukraine and do more to prevent arms flowing into Ukraine across porous Russian borders, she said. "If all this does not stop," she told Parliament, "then we will not shy away from imposing new sanctions."

But the French government repeated its refusal to cancel the warship sale, saying it would be illegal to break a contract under international law. French officials view the economic imperative outweighing the geopolitical costs. "France cannot bat aside these economic questions with the back of a hand," said Jean Carrère, who leads the French Senate's committee on foreign relations and defense, noting "the serious economic difficulties" the country is facing.

Even within the Obama administration, there are cross pressures about how to respond, most recently over sending more troops to bolster the security of Poland and other NATO allies in the east. Mr. Obama sided with aides who advised against a more robust military presence in the east in the short term, for fear that it would be unnecessarily provocative. But he did promise to spend as much as $1 billion, if approved by Congress, to increase joint exercises, expand military training and pre-position equipment.

In a speech Wednesday in Warsaw's Castle Square before flying to Belgium, he had strong words of support. "Poland will never stand alone," Mr. Obama declared. "Estonia will never stand alone. Latvia will never stand alone. Lithuania will never stand alone. Romania will never stand alone."

Mr. Obama was addressing a 25th anniversary ceremony for the Polish elections that led to the eventual collapse of Communist rule, and he linked those stirring events to the current Ukraine upheaval. "The Ukrainians of today are the heirs of Solidarity -- men and women like you who dared to challenge a bankrupt regime," he said, referring to the Polish trade union group.

His message drew mixed reactions. "The U.S. president did not miss the opportunity to say the right things in the right time and place," said Wlodzimierz Cimoszewicz, a former Polish prime minister.

But Witold Waszczykowski, a senior member of Poland's chief opposition party, Law and Justice, said $1 billion for a region of 10 allies is not much. "In the past 25 years, we heard a lot of nice speeches, also from other American presidents," he said. "Now, it's time to turn words into deeds. Because it's us who are neighbors with the greatest aggressor."