The White House on Tuesday said Russia so far appears to be fulfilling a decision to withdraw troops from Syria, but cautioned it was too soon to judge the impact.
"The earliest indications are that the Russians are following through," said White House spokesman Josh Earnest, as the first of Russia's warplanes to leave Syria received a hero's welcome back home.
The Pentagon reported it had seen "some Russian aircraft depart Syria and return to Russia" although that did not amount to "a large contingent of Russian forces."
US officials have been cautious about their assessment of Putin's motives following a sudden announcement Monday.
"Russia did not give direct advance notice" of the announcement, Earnest said, although a telephone call with Putin was pre-planned.
The White House cautioned "it is still too early at this point to determine the impact that might have on the broader situation."
US officials will be keen to probe any weakening of Russia's support for Assad.
Putin launched air strikes against rebel positions in September followed by a massive troop deployment that turned the tide of a long and brutal war in Syrian President Bashar al-Assad's favor, rescuing his regime from the brink of collapse.
Earnest noted that there had been tension in Russia's support for a negotiated end to the conflict and its military support for Assad.
Moscow has rejected demands from the opposition and from the White House that peace would mean Assad stepping aside.
Addressing Russia's withdrawal, Earnest said it was "too early to assess what impact this will have on the Syrian government's negotiating position" in peace talks.