WASHINGTON — President Obama tried to mend fences with Chancellor Angela Merkel of Germany on Friday, calling her “one of my closest friends on the world stage.” But Ms. Merkel replied tartly that Germany still had significant differences with the United States over surveillance practices and that it was too soon to return to “business as usual.”

The cordial but slightly strained encounter, which played out as the two leaders stood next to each other at a Rose Garden news conference, attested to the lingering scars left by the sensational disclosure last October that the National Security Agency had eavesdropped on Ms. Merkel’s phone calls.

It came as the two leaders sought to project a unified front against Russia’s aggression in Ukraine, threatening President Vladimir V. Putin with sweeping new sanctions if Russia disrupted elections in Ukraine later this month, even as they acknowledged that not all European countries were ready to sign on to the most punishing measures.

Ms. Merkel, who last fall declared that “spying between friends is simply unacceptable” and that the United States had opened a breach of trust that would have to be repaired, said at the news conference that “we have a few difficulties yet to overcome.” One remaining issue, she said, was the “proportionality” of the surveillance.