SIMFEROPOL, Crimea — Russia said Monday it was pulling an armored battalion of about 500 troops away from the Ukrainian border but kept tens of thousands in place, prompting a worried response from the Kiev government about what the United States warned was still a ‘‘tremendous buildup.’’
Russia moved quickly to strengthen its economic hold on Crimea, with Prime Minister Dmitry Medvedev arriving in this newly annexed peninsula with promises of funds for improved power supplies, water lines, education, and pensions for the elderly.
Russia’s takeover of the strategic Black Sea region, its troop buildup near Ukraine’s border, and its attempts to compel constitutional changes in Ukraine have markedly raised tensions with the West and prompted fears Moscow intends to invade other areas of its neighbor.
President Vladimir Putin of Russia told Chancellor Angela Merkel of Germany in a phone call Monday that some troops were being withdrawn from the Ukraine border, Merkel’s office said. The Russian Defense Ministry confirmed the action but did not say whether it signaled the start of a broader pullback.
The United States reacted cautiously to the troop movement, with Secretary of Defense Chuck Hagel saying that tens of thousands of Russian forces still remained along the Ukrainian border, a situation he called ‘‘a tremendous buildup.’’
The new government in Ukraine said the action only increased its uneasiness about Russia’s intentions.
‘‘We have information that Russia is carrying out incomprehensible maneuvers on the border with Ukraine,’’ Ukrainian Foreign Ministry spokesman Yevgen Perebyinis said. ‘‘Troops in some places are moving backward, some of them are moving forward.
“Which is why, obviously, we are worried by these movements of armed forces. We have no clear explanation from the Russian side about the aim of these movements.’’
Foreign Minister Sergey Lavrov of Russia and Secretary of State John Kerry also discussed Ukraine by phone Monday, a day after holding talks in Paris, the ministry said.
A senior US official said Lavrov had promised Kerry that a division of Russian troops would be pulled back; a division generally consists of thousands of troops.
‘‘Now there have been reports of possible drawdowns of Russian military forces from the border. We haven’t seen that yet, but if they turn out to be accurate, that would be a good thing,’’ said White House spokesman Jay Carney.
Concerns of a possible invasion of eastern Ukraine were stoked by the large numbers of troops Russia had along the Ukrainian border for what Moscow said were military exercises.
One Russian battalion that had been sent to the Rostov region next to Ukraine was being withdrawn to its permanent base in the central Samara region, Russian news agencies quoted the Defense Ministry as saying Monday.
Alexander Rozmaznin, deputy chief of the Ukrainian armed forces command center, confirmed a drop in Russian troop numbers — about 500 soldiers — along the border.
In Kiev, Ukraine’s acting president flatly rejected escalating Russian pressure to turn Ukraine into a loose federation.
‘‘Russia’s leadership should deal with problems in the Russian Federation, and not with Ukraine’s problems,’’ Ukraine’s acting president, Oleksandr Turchinov, said. ‘‘It is Ukrainians that should dictate the form of the new constitution and how the country is structured.’’
Medvedev, who led a delegation of Cabinet ministers on a surprise visit to Crimea, pledged that Russia will quickly boost salaries and pensions there and pour in resources to improve education, health care, and local infrastructure.
Russia annexed Crimea from Ukraine in March after a hastily called referendum held just two weeks after Russian forces had taken control the Black Sea region. Ukraine and the West have rejected the vote.
The annexation came after Ukraine’s president, Viktor Yanukovych, was ousted in February and fled to Russia after months of protests. Russia claims the ouster was a coup and that the new Ukrainian authorities are fascists who will abuse Ukraine’s large ethnic Russian population.
The United States and the European Union have imposed travel bans and asset freezes on members of Putin’s inner circle for the annexation and warned that Russia will face even more painful sanctions if it tries to invade eastern Ukraine.
To keep its influence over eastern and southern Ukraine, Russia has pushed for Ukraine to become a federation where regions would have broad powers. The United States says it is up to Ukrainians to determine the structure of their government, not Moscow.
Medvedev said Russia will create a special economic zone in Crimea — a peninsula of 2 million people — that will create incentives for business with lower taxes and simpler rules. Russia will also seek to develop the region as a top tourist destination.
Medvedev particularly emphasized the need to ensure a stable power supply for the peninsula. Crimea currently gets about 80 percent of its electricity and a similar share of its water from Ukraine, and power cutoffs last week raised fears that the Ukrainian government could use energy as a weapon to bargain with Russia.